#include <bits/stdc++.h>
#define ll long long
#define blugy %1000000007
ll lowkey = 1e9+7;
using namespace std;
int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    ll n, k, i, mer, mar=0, sum=1, p1, p2, p;
    cin >> n >> k;
    k--;
    vector<ll> t(n), dp(n,0), cucc(n,0),s(n);
    cin >> t[0];
    s[0]=t[0];
    for(i=1; i<n; i++)
    {
        cin >> t[i];
        s[i]=s[i-1]+t[i];
    }
    for(i=0; i<k; i++)if(2*s[i]<=s[k])sum=(sum<<1)blugy;
    cucc[k]=sum;
    dp[k]=sum;
    for(i=k+1; i<n; i++)
    {
        p1=k+1;
        p2=i;
        while(p1!=p2)
        {
            p=(p1+p2)/2;
            if(2*s[p]>s[i])p2=p;
            else p1=p+1;
        }
        if(s[i-1]>t[i])dp[i]=cucc[i-1]-cucc[p1-2];
        if(dp[i]<0)dp[i]+=lowkey;
        dp[i]=dp[i]blugy;
        cucc[i]=cucc[i-1]+dp[i];
        if(cucc[i]<0)cucc[i]+=lowkey;
        cucc[i]=cucc[i]blugy;
    }
    cout << (2*dp[n-1])blugy;
}