#include <iostream>
#include <fstream>
using namespace std;
ifstream f("deque.in");
ofstream g("deque.out");

int main()
{int n,k,i,x[100],minn,s=0;
    f>>n>>k;
    for(i=1;i<=n;i++)
        f>>x[i];
for(i=1;i<=n-k+1;i++)
{minn=x[i];
    if(x[i+1]<minn)
        minn=x[i+1];
     if(x[i+2]<minn)
            minn=x[i+2];
     s=s+minn;


}
       g<<s;
        return 0;
}