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