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