#include <bits/stdc++.h>
#define DIM 501
using namespace std;

ifstream fin("deque.in");

ofstream fout("deque.out");

int n,k,x,cnt, Min = 1e9,s;

deque<int> D;

int main(){

    fin >> n >> k;

    for(int i=1;i<=n;i++){

        if(cnt == 3){

            s+=Min;

            D.pop_front();

            cnt--;

            Min = min(D.front(),D.back());

        }

        fin >> x;

        if(x < Min)

            Min = x;

        D.push_back(x);

        cnt++;

    }

    if(cnt == 3){

        s+=Min;

        D.pop_front();

        cnt--;

        Min = min(D.front(),D.back());

    }

    fout << s;

    return 0;
}