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