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