#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];
  auto ans = 0;
  for (int i = 1; i < N; ++i) {
    int MaxNum = max(A[i - 1], A[i]);
    ans += MaxNum - A[i];
    A[i] = MaxNum;
  }
  cout << ans << endl;
  return 0;
}
