#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int n;
  cin >> n;
  long long int a[n];
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  int sum = 0;
  int mx = a[0];
  for (int i = 1; i < n; i++) {
    if (a[i] > mx) {
      mx = a[i];
    } else {
      sum += mx - a[i];
      a[i] = mx;
    }
  }
  cout << sum;
  return 0;
}
