#include <fstream>

using namespace std;

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

int n, k, v[5000000];
long long sum, val;

int main()
{
    fin>>n>>k;
    for(int i = 1; i <= n; i++){
        fin>>v[i];
    }
    for(int i = 1; i <= n - k + 1; i++){
        val = min(min(v[i], v[i + 1]), v[i + 2]);
        sum += val;
    }
    fout<<sum;
    return 0;
}