#include <iostream>
#include <fstream>
using namespace std;

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

void stergere(long n, int v[]) {
    for (int i = 0; i < n; ++i)
        v[i] = v[i+1];
}

long minim (long n, int v[]) {
    int aux = v[0];
    for (long i = 1; i < n; ++i)
        if (aux > v[i])
            aux = v[i];
    return aux;
}

int main() {
    int m, n, s = 1, dr = 0;
    long long s = 0;

    in >> m >> n;

    long long v[m];
    int deq[n];

    for (int i = 0; i < m; ++i) {
        in >> v[i];

        while (st <= dr && v[i] <= v[deq[dr]])
            dr--;

        dr++; deq[dr] = i;
        if (deq[st] < i - n + 1) st++;
        if (i >= n - 1)          s += v[deq[st]];
    }

    out << s;

    in.close();
    out.close();
    return 0;
}