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