#include <bits/stdc++.h>
using namespace std;

int n, k;
int a[5000005];
int dq[5000005];
int main(){
    freopen("deque.in", "r", stdin);
    freopen("deque.out", "w", stdout);
    scanf("%d%d", &n, &k);
    int Front = 1, Back = 0;
    long long Sum = 0;
    for(int i = 1; i < k ; ++i){
        scanf("%d", &a[i]);
        while(Back >= Front && a[dq[Back]] > a[i]) --Back;
        dq[++Back] = i;
    }
    for(int i = k; i <= n ; ++i){
        scanf("%d", &a[i]);
        while(Back >= Front && a[dq[Back]] > a[i]) --Back;
        dq[++Back] = i;
        Sum = Sum + a[dq[Front]];
        if(dq[Front] <= i - k + 1) ++Front;
    }
    printf("%lld", Sum);
    return 0;
}