#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, c, d, tmp, max;
  string s;
  long long ans;
  a = b = c = d = ans = tmp = 0;
  cin >> a;
  vector<int> x(a);
  for (int i = 0; i < a; i++) {
    cin >> x[i];
  }
  for (int i = 1; i < a; i++) {
    if (x[i] < x[i - 1]) {
      ans += x[i - 1] - x[i];
    }
  }
  cout << ans;
}
