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