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