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