#include <bits/stdc++.h>
using namespace std;

#define int long long

int n, k;


signed main() {
	cin>>n>>k;
  vector<long long> a(n+1);
  vector<long long> s(n+1);
  map<int, int> m;
  long long eredmeny=LLONG_MAX/2;
  for(int i=1; i<=n; i++) {
    cin>>a[i];
    a[i]+=i;
    s[i]=s[i-1]+a[i];
    if(i<k) {
      m[a[i]]+=1;
    }
  }
  cout<<endl;
  for(int vege=k; vege<=n; vege++) {
    m[a[vege]]+=1;
    
    auto it=m.rbegin();
    eredmeny=min(eredmeny, (it->first)*k-(s[vege]-s[vege-k]));
    m[a[vege-k+1]]-=1;
    if(m[a[vege-k+1]]==0) {
      m.erase(a[vege-k+1]);
    }
  }
  cout<<eredmeny<<endl;
  
	return 0;
}