#include <bits/stdc++.h>
using namespace std;
using Graph = vector<vector<int>>;
const long long MOD = 998244353;
int main(void) {
  int n;
  cin >> n;
  int max = 0, ans = 0;
  for (int i = 0; i < (int)(n); i++) {
    int a;
    cin >> a;
    if (a < max)
      ans += max - a;
    else
      max = a;
  }
  cout << ans << endl;
  return 0;
}
