#include <fstream>
#include <deque>
#include <algorithm>
using namespace std;

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

int main()
{
    ios_base::sync_with_stdio(0);
    in.tie(0);

    int n, k;
    in >> n >> k;

    long long sum = 0LL;
    deque<int> dq;

    for(int i = 0; i < n; i++)
    {
        int tmp;
        in >> tmp;

        dq.push_back(tmp);

        if(dq.size() == 3)
        {
            sum += min({dq[0],dq[1],dq[2]});
            dq.pop_front();
        }
    }

    out << sum;

    return 0;
}