#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  cin >> N;
  vector<int64_t> A(N);
  for (int i = 0; i < N; i++) {
    cin >> A.at(i);
  }
  int ans = 0;
  int M = A.at(0);
  for (int i = 1; i < N; i++) {
    if (M > A.at(i)) {
      ans = ans + (M - A.at(i));
    } else {
      M = A.at(i);
    }
  }
  cout << ans << endl;
}
