#include <iostream>
#include <fstream>

using namespace std;
ifstream in("deque.in");
ofstream out("deque.out");
int n,i,p,j,v[5000001];
int mi;
long long s;
int main()
{
    in>>n>>p;
    for(i=1;i<=n;i++) in>>v[i];

    s=0;
    for(i=1;i<=n-p+1;i++)
    {
        mi=10000001;
        for(j=0;j<p;j++)
        {
            if(v[i+j]<mi) mi=v[j+i];
        }
        s=s+mi;
    }
    out<<s;

    return 0;
}