#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  cin >> N;
  vector<int> A(N);
  for (int i = 0; i < N; ++i) {
    cin >> A[i];
  }
  int ans = 0;
  int max = A[0];
  for (int i = 1; i < N; ++i) {
    if (max > A[i]) {
      ans += max - A[i];
    } else {
      max = (max < A[i]) ? A[i] : max;
    }
  }
  cout << ans << endl;
  return 0;
}
