#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long n;
  cin >> n;
  vector<long long> v(n);
  for (long long i = 0; i < n; i++) cin >> v[i];
  long long int mx = v[0];
  long long int sum = 0;
  for (long long i = 1; i < n; i++) {
    if (mx > v[i]) {
      sum += (mx - v[i]);
    } else {
      mx = v[i];
    }
  }
  cout << sum;
}
