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