#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1000000007;
const long long INF = 9999999999999999;
using ll = long long;
int main() {
  int N;
  cin >> N;
  int a[N];
  for (int i = 0; i < N; i++) {
    cin >> a[i];
  }
  int ans = 0;
  for (int i = 1; i < N; i++) {
    if (a[i] < a[i - 1]) {
      ans += a[i - 1] - a[i];
      a[i] = a[i - 1];
    }
  }
  cout << ans << endl;
  return 0;
}
