#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, fumi = 0;
  for (auto& a : A) {
    if (fumi <= a) {
      fumi = a;
    } else {
      ans += fumi - a;
      a = fumi;
    }
  }
  cout << ans << endl;
  return 0;
}
