#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  cin >> N;
  int A[N];
  int max = 0, ans = 0;
  for (int i = 0; i < N; i++) {
    cin >> A[i];
    if (A[i] < max)
      ans += max - A[i];
    else {
      max = A[i];
    }
  }
  cout << ans << endl;
  return 0;
}
