11683 - Laser Sculpture

/*
 *
 * problem link: https://onlinejudge.org/index.php?option=onlinejudge&Itemid=8&page=show_problem&category=0&problem=2730&mosmsg=Submission+received+with+ID+27781398
 *
 * */

#include <bits/stdc++.h>

using namespace std;

#define ll long long
#define endl '\n'
#define sz(x) int(x.size())
#define all(x) x.begin(), x.end()

int main() {
    ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);

    int h, w;
    while (scanf("%d", &h), h) {
        scanf("%d", &w);
        int high[w + 1];
        high[0] = h;
        stack<int> stk;
        stk.push(0);
        int cnt = 0;
        for (int i = 1; i <= w; i++) {
            scanf("%d", &high[i]);
            while (stk.size() && high[stk.top()] <= high[i]) stk.pop();
            if (stk.size() && high[stk.top()] > high[i]) {
                cnt += high[stk.top()] - high[i];
                stk.pop();
            }
            stk.push(i);
        }
        cout << cnt << endl;
    }
}