#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  cin >> N;
  int sum = 0, A[N];
  for (int i = 0; i < N; i++) {
    cin >> A[i];
    if (i == 0) continue;
    if (A[i] < A[i - 1]) {
      sum += (A[i - 1] - A[i]);
      A[i] = A[i - 1];
    }
  }
  cout << sum << endl;
}
