#include <iostream>
using namespace std;
int a[300000] = { 0 };
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
int n, max = 0;
while (cin >> n)
{
if (n > max) max = n;
a[n]++;
}
for(int i=1;i<=max;i++)
if (a[i] == 2)
{
cout << i;
break;
}
}
我一直想不透