#include <fstream>
using namespace std;
ifstream f("deque.in");
ofstream g("deque.out");
int n,k,a,b,c,d,c1,c2,s;
int main()
{
    f>>n>>k;
    f>>a>>b>>c;
    n-=3;
    s+=min(a,min(b,c));
    c1=b;
    c2=c;
    while(n)
    {
        f>>d;
        s+=min(c1,min(c2,d));
        c1=c2;
        c2=d;
        n--;
    }
    g<<s;
}