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