#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 ans = 0;
  for (int i = 1; i < N; i++) {
    int diff = A[i - 1] - A[i];
    if (diff > 0) {
      ans += diff;
      A[i] += diff;
    }
  }
  cout << ans << endl;
  return 0;
}
