#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;
int main() {
  int n;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < (n); ++i) cin >> a[i];
  int cnt = 0;
  int max = a[0];
  for (int i = 0; i < (n - 1); ++i) {
    if (max > a[i + 1]) {
      cnt += max - a[i + 1];
    } else if (max < a[i + 1]) {
      max = a[i + 1];
    }
  }
  cout << cnt << endl;
  return 0;
}
