#include <iostream>
 
using namespace std;
 
int main() {
  int n; cin >> n;
  long long sum = 0;
  long long max = 0;
  for (int i = 0; i < n; i++) {
    long long a; cin >> a;
    if (a > max) max = a;
    else sum += max - a;
  }
  cout << sum << endl;
  
}