#include <cstdio>

#define minim(a,b) ((a < b) ? a : b)

using namespace std;

int n,k;

int a[10000000];

long long sum;

int main()
{
    int a1,a2;
    freopen("deque.in","r",stdin);
    freopen("deque.out","w",stdout);
    scanf("%i %i",&n,&k);
    for(int i=1;i<=n;i++)
        scanf("%i",&a[i]);
    for(int i=1;i<=n-k+1;i++)
    {
        a1=minim(a[i] , a[i+1]);
        a2=minim(a1 , a[i+2]);
        sum += a2;
    }
    printf("%i",sum);
    return 0;
}