#include <iostream>
#include <fstream>
using namespace std;

int main()
{
    int n, k;
    ifstream fin("deque.in");
    ofstream fout("deque.out");

    fin >> n >> k;

    int a[n];

    for (int i = 0; i < n; i++)
    {
        fin >> a[i];
    }

    int totalSuma = 0;

    for (int i = 0; i < n - 2; i++)
    {
        int minim = min(a[i], min(a[i + 1], a[i + 2]));
        totalSuma += minim;
    }

    fout << totalSuma;

    fin.close();
}