#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.at(i);
  int highest = A.at(0);
  int sum = 0;
  for (int i = 1; i < N; ++i) {
    if (A.at(i) < highest)
      sum += highest - A.at(i);
    else
      highest = A.at(i);
  }
  cout << sum;
  return 0;
}
