#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int ans = 0;
  int a0 = 0;
  for (int i = 0; i < n; ++i) {
    int a;
    cin >> a;
    if (a < a0) {
      ans += a0 - a;
      a = a0;
    }
    a0 = a;
  }
  cout << ans << endl;
  return 0;
}
