code

#include<iostream>
#include<stack>
using namespace std;
int getMaxArea(long long hist[], long long n){
    stack<int> s;
    int max_area = 0;
    int tp;
    int area_with_top;
    int i = 0;
    while (i < n){
        if (s.empty() || hist[s.top()] <= hist[i])
            s.push(i++);
        else{
            tp = s.top();
            s.pop();
            area_with_top = hist[tp] * (s.empty() ? i :
                                   i - s.top() - 1);
        if (max_area < area_with_top)
                max_area = area_with_top;
        }
    }
    while (s.empty() == false){
        tp = s.top();
        s.pop();
        area_with_top = hist[tp] * (s.empty() ? i :
                                i - s.top() - 1);
        if (max_area < area_with_top)
            max_area = area_with_top;
    }
    return max_area;
}
int main() {
    long long n;
    cin>>n;
    long long m;
    cin>>m;
    long long w[m];
    long long hist[m];
    for(int i=0;i<m;i++){
        cin>>w[i];
        hist[i] = n - w[i];
    }
    cout<<getMaxArea(hist, m);
    return 0;
}