#include <iostream>
#include <fstream>

using namespace std;

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

const int dim = 5000005;

int n,m,rmq[20][dim],dif,log2,st,dr,pu,rasp,mij;

int cauta(int x)
{
    st = 1,mij,pu;
    dr = 30,rasp = -1;
    while (st <= dr)
    {
        mij = (st+dr)/2;
        pu = (1<<mij);
        if (pu == x)
        {
            return mij;
        }
        else if (pu < x)
        {
            st = mij+1;
            rasp = mij;
        }
        else
        {
            dr = mij-1;
        }
    }
    return rasp;
}

int rezolva(int x,int y)
{
    dif = y-x+1;
    log2 = cauta(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;
    }
    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 s = 0;
    for (int i=1; i+m-1<=n; i++)
    {
        s += rezolva(i,i+m-1);
    }
    out << s;
    return 0;
}