#include <fstream>

int main(){
    int n, k;
    std::ifstream in("deque.in");
    long long int result = 0;
    in>>n>>k;
    std::pair<int, int> last;
    in>>last.first>>last.second;
    int tmp, minn = std::min(last.first, last.second);
    for(int x = 2;x<n;x++){
        in>>tmp;
        result+=std::min(minn, tmp);
        last.first = last.second;
        last.second = tmp;
        minn = std::min(last.first, last.second);
    }
    std::ofstream("deque.out")<<result;
}