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