#include<bits/stdc++.h>
#define in f
#define out g
using namespace std;

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


int n;
int k;
int v[5000001];
int aux;
int x;
int y;


int main() {

    in >> n;
    in >> k;

    for(int i = 1; i <= n; i++) {
        in >> v[i];
    }
    for(int i = 1; i <= n - k + 1; i++) {
        x = min(v[i], v[i+1]);
        y = min(v[i+1], v[i+2]);
        aux = aux + min(x, y);
    }
    out << aux;
}