#include <bits/stdc++.h>

using namespace std;

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

int  n, k, minim = 10000002;
long long s;
int arr[5000005];

int main()
{
    in>>n>>k;
    for(int i = 1; i <= n; i++)
        in>>arr[i];

    for(int i = 1; i <= n - 2; i++){
        for(int j = 0; j <= 2; j++)
            if(minim > arr[i + j])
                minim = arr[i + j];
        s += minim;
        minim = 10000002;
    }

    out<<s;

    return 0;
}