#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < n; i++) {
    int val;
    cin >> val;
    a[i] = val;
  }
  int height = 0;
  for (int i = 0; i < n - 1; i++) {
    if (a[i + 1] >= a[i]) {
      continue;
    }
    int stool = a[i] - a[i + 1];
    a[i + 1] += stool;
    height += stool;
  }
  cout << height;
}
