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