#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 s = 0;
  for (int i = 0; i < (n - 1); ++i) {
    if (a[i + 1] < a[i]) {
      s += a[i] - a[i + 1];
      a[i + 1] = a[i];
    }
  }
  cout << s << endl;
  return 0;
}
