#include<iostream>
#include<fstream>
using namespace std;
int minim(int a,int b,int c)
{
    if(a<b && a<c)
        return a;
    else
    if(b<a && b<c)
        return b;
    else
        return c;
}
int main()
{
    ifstream f("deque.in");
    ofstream g("deque.out");
    long n,k,i=3,x,y,z,s=0;
    f>>n>>k;
    f>>x>>y>>z;
    s=s+minim(x,y,z);
    while(i<n)
    {
        i++;
        if(i<=n)
        {
            f>>x;
            s=s+minim(x,y,z);
        }
        else
            break;
        i++;
        if(i<=n)
        {
            f>>y;
            s=s+minim(x,y,z);
        }
        else
            break;
        i++;
        if(i<=n)
        {
            f>>z;
            s=s+minim(x,y,z);
        }
    }
    g<<s;
    return 0;
}