#include <bits/stdc++.h>

using namespace std;
ifstream in ("deque.in");
ofstream out ("deque.out");

queue <long long> dqIg;

int main()
{
    long long n, k;
    in >> n >> k;

    for(int i = 0; i < n; i++){
        long long crt;
        in >> crt;
        dqIg.push(crt);
    }

    long long suma = 0;
    long long prev2x = dqIg.front();
    dqIg.pop();
    long long prev1x = dqIg.front();
    dqIg.pop();
    long long crt;

    while(!dqIg.empty()){
        crt = dqIg.front();
        dqIg.pop();
        long long minimum = min(prev1x, min(prev2x, crt));
        suma += minimum;
        prev2x = prev1x;
        prev1x = crt;
    }
    out << suma;
    return 0;
}