usaco-practice/cses/Increasing Array/index.cpp

#include <iostream>
using namespace std;

int main() {
  int n;
  cin >> n;
  int px = 0;
  long long ans = 0;
  for (int i = 0; i < n; i++) {
    int x;
    cin >> x;
    if (x < px) {
      ans += px - x;
      x = px;
    }
    px = x;
  }
  cout << ans;
}