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