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