#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  vector<int> v(n);
  for (int i = 0; i < n; i++) cin >> v[i];
  int sm = 0, mx = v[0];
  for (int i = 1; i < n; i++) {
    if (v[i] >= mx) {
      mx = v[i];
    } else {
      sm += mx - v[i];
    }
  }
  cout << sm << "\n";
}
