#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < n; i++) {
    int val;
    cin >> val;
    a[i] = val;
  }
  int sum = 0;
  for (int i = 1; i < n; i++) {
    if (a[i] >= a[i - 1]) {
      continue;
    }
    int diff = a[i - 1] - a[i];
    a[i] += diff;
    sum += diff;
  }
  cout << sum;
}
