#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int n;
  cin >> n;
  vector<int> a(n), r(n);
  int result = 0;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  for (int i = 1; i < n; i++) {
    int ans = a[i - 1] - a[i];
    if (ans > 0) {
      r[i] = ans;
      a[i] += ans;
    } else {
      r[i] = 0;
    }
  }
  for (int i : r) {
    result += i;
  }
  cout << result;
}
