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