#include <vector>
#include <string>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <queue>
#include <bitset>
#include <stack>
#include <list>
#include <numeric>
#include <algorithm>
#include <random>
#include <chrono>
#include <cstdio>
#include <fstream>
#include <iostream>
#include <sstream>
#include <iomanip>
#include <climits>
#include <cctype>
#include <cmath>
#include <ctime>
#include <cassert>
using namespace std;
#define ULL unsigned long long
#define LL long long
#define PII pair <int, int>
#define VB vector <bool>
#define VI vector <int>
#define VLL vector <LL>
#define VD vector <double>
#define VS vector <string>
#define VPII vector <pair <int, int> >
#define VVI vector < VI >
#define VVB vector < VB >
#define SI set < int >
#define USI unordered_set <int>
#define MII map <int, int>
#define UMII unordered_map <int, int>
#define MS multiset
#define US unordered_set
#define UM unordered_map
#define UMS unordered_multiset
#define UMM unordered_multimap
#define FORN(i, n) for(int i = 0; i < (n); ++i)
#define FOR(i, a, b) for(int i = (a); i <= (b); ++i)
#define FORD(i, a, b) for(int i = (a); i >= (b); --i)
#define MX(x, y) x = max(x, y);
#define MN(x, y) x = min(x, y);
#define SZ size()
#define BG begin()
#define EN end()
#define CL clear()
#define X first
#define Y second
#define RS resize
#define PB push_back
#define MP make_pair
#define ALL(x) x.begin(), x.end()
#define INS insert
#define ER erase
#define CNT count
template <class T> ostream& operator <<(ostream & os, const vector<T> &vec)
{
for (int i = 0; i < vec.size() - 1; ++i) os << vec[i] << ' ';
return os << vec[vec.size() - 1];
}
template <class T1, class T2> ostream& operator <<(ostream & os, const pair<T1, T2> &p)
{
return os << p.X << " " << p.Y;
}
template <typename T>
void pr(T var1)
{
cout << var1 << '\n';
}
template <typename T, typename... Types>
void pr(T var1, Types... var2)
{
cout << var1;
pr(var2...);
}
void in(int &n, VI &a) //array of ints
{
cin >> n;
a.CL, a.RS(n + 1);
FOR(i, 1, n) cin >> a[i];
}
void in(int &n, VS &a) //array of strings
{
cin >> n;
a.CL, a.RS(n + 1);
FOR(i, 1, n) cin >> a[i];
}
void in(int &n, VPII &a) //array of pairs
{
cin >> n;
a.CL, a.RS(n + 1);
FOR(i, 1, n) cin >> a[i].X >> a[i].Y;
}
void in(int &n, int &m, VVI &g) //unweighted graph
{
cin >> n >> m;
g.CL, g.RS(n + 1);
FOR(i, 1, n)
{
int x, y;
cin >> x >> y;
g[x].PB(y);
g[y].PB(x);
}
}
void in(int &n, VVI &g) //unweighted tree
{
cin >> n;
g.CL, g.RS(n + 1);
FOR(i, 1, n - 1)
{
int x, y;
cin >> x >> y;
g[x].PB(y);
g[y].PB(x);
}
}
void in(int &n, int &m, vector <VPII> &g) //weighted graph
{
cin >> n >> m;
g.CL, g.RS(n + 1);
FOR(i, 1, n)
{
int x, y, z;
cin >> x >> y >> z;
g[x].PB({y, z});
g[y].PB({x, z});
}
}
void in(int &n, vector <VPII> &g) //weighted tree
{
cin >> n;
g.CL, g.RS(n + 1);
FOR(i, 1, n - 1)
{
int x, y, z;
cin >> x >> y >> z;
g[x].PB({y, z});
g[y].PB({x, z});
}
}
const int MOD = 1e9 + 7;
int n, k;
VI a, pow2;
vector <ULL> ps;
int main()
{
#ifdef AT_HOME
freopen("a.in", "r", stdin);
freopen("a.out", "w", stdout);
#endif
cin >> n >> k;
if (n == 1)
{
pr(2);
return 0;
}
a.RS(n + 1);
ps.RS(n + 1);
pow2.RS(n + 1);
FOR(i, 1, n)
{
cin >> a[i];
ps[i] = ps[i - 1] + a[i];
}
pow2[0] = 1;
FOR(i, 1, n) pow2[i] = pow2[i - 1] * 2 % MOD;
ULL solLeft = 0, solRight = 0;
ULL larvaSize = a[k];
//Bimbo balra kell induljon, hogy tulelje (kiveve ha utolso)
FORD(i, k - 1, 2)
{
//Ha i balra indult
if (larvaSize >= ps[i]) solLeft = (solLeft + pow2[i - 1]) % MOD;
//Ha i jobbra indult
larvaSize += a[i];
}
//Az elso mindegy merre indult
if (k > 1) larvaSize += a[1], solLeft += 2;
VI dp(n + 1); //hany lehetoseg van ha i-1-el bezarolag jobbra indulunk
VI dpSuf(n + 2);
dpSuf[n] = dp[n] = 2;
//Bimbo ezutan talalkozik a "jobboldallal"
FORD(i, n - 1, k + 1)
{
larvaSize = ps[i - 1];
//Ha i jobbra indult
if (larvaSize > ps[n] - ps[i - 1]) //Meg tudunk enni mindenkit aki maradt
{
dp[i] = (dp[i + 1] + pow2[n - i]) % MOD;
}
else
{
//Meddig tudjuk osszeszedni a jobbra levo larvakat?
int l = i, r = n;
while (r - l > 1)
{
int m = (l + r) / 2;
LL s = ps[m] - ps[i - 1];
if (larvaSize > s) l = m;
else r = m;
}
LL aux = dpSuf[i + 1];
aux -= dpSuf[r + 1];
if (aux < 0) aux += MOD;
dp[i] = (dp[i] + aux) % MOD;
}
dpSuf[i] = (dpSuf[i + 1] + dp[i]) % MOD;
//Ha i tul nagy
if (larvaSize <= a[i])
{
pr(0);
return 0;
}
}
solRight = dp[k + 1];
if (k < n)
if (ps[n - 1] <= a[n])
{
pr(0);
return 0;
}
else ;
else solRight = 2; //ha utolso, indulhatott jobbra is es minden ugyanaz
pr(solLeft % MOD * solRight % MOD);
return 0;
}