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