#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  cin >> N;
  int A[N];
  int s = 0;
  for (int i = 0; i < N; ++i) {
    cin >> A[i];
    if (i == 0) continue;
    if (A[i - 1] > A[i]) {
      s += (A[i - 1] - A[i]);
      A[i] += (A[i - 1] - A[i]);
    }
  }
  cout << s << endl;
}
