#include <string>
#include <vector>
#include <list>
#include <cmath>
#include <algorithm>
#include <map>
#include <set>
#include <iostream>
#include <sstream>
#include <cstdio>
#include <cstdlib>
using namespace std;

class Solution {
public:
  int singleNonDuplicate(vector <int> &nums);
};

int Solution::singleNonDuplicate(vector <int> &nums)
{
  return 0;
}

int main()
{
  int i;
  vector <int> n;
  Solution s;

  while (cin >> i) n.push_back(i);

  cout << s.singleNonDuplicate(n) << endl;
}