#include <iostream>
#include <fstream>
using namespace std;
ifstream fin("deque.in");
ofstream fout("deque.out");
int main() {
    int n, k;
    fin>>n>>k;
    int v[4], s = 0;
    fin>>v[1];
    fin>>v[2];
    for (int i = 0; i < n - 2; ++i) {
        v[0] = v[1];
        v[1] = v[2];
        fin>>v[2];
        if (v[0] >= v[1] && v[2] >= v[1])
            s = s + v[1];
        else if (v[1] >= v[0] && v[2] >= v[0])
            s = s + v[0];
        else if (v[0] >= v[2] && v[1] >= v[2])
            s = s + v[2];
    }
    fout<<s;
    fin.close();
    fout.close();
    return 0;
}