#include <iostream>
#include <fstream>

using namespace std;

ifstream in("deque.in");
ofstream out("deque.out");

const int dim = 5000005;

int n,m,rmq[21][dim],dif,log2,st,dr,pu,rasp,mij,lg[dim];

int rezolva(int x,int y)
{
    dif = y-x+1;
    log2 = lg[dif];
    return min(rmq[log2][x] , rmq[log2][y - (1<<log2) + 1]);
}

int main()
{
    in >> n >> m;
    int a,i,j;
    for (i=1; i<=n; i++)
    {
        in >> a;
        rmq[0][i] = a;
    }
    lg[1] = 0;
    for (i=2; i<=n; i++)
    {
        lg[i] = 1 + lg[i/2];
    }
    for (j=1;  (1<<j) <= n; j++)
    {
        for (i=1; i + (1<<j) - 1 <=n; i++)
        {
            rmq[j][i] = min(rmq[j-1][i] , rmq[j-1][ i + (1<<(j-1))]);
        }
    }
    long long int maxi = -30006,rez,stmax,drmax,s = 0;
    for (i=1; i+m-1<=n; i++)
    {
        rez = rezolva(i,i+m-1);
        s += rez;
    }
    out << s;
    return 0;
}