#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  {
    int n;
    cin >> n;
    int a[n];
    for (int i = 0; i < n; i++) cin >> a[i];
    int max = a[0], c = 0;
    for (int i = 1; i < n; i++) {
      if (a[i] < max)
        c += (max - a[i]);
      else
        max = a[i];
    }
    cout << c;
  }
  return 0;
}
