1036.The Lost Card

#include <cstdio>
#include <cstdlib>
#include <iostream>
using namespace std;

int main(int argc, char* argv[]) {
    int n, i, s = 0;

    scanf("%d", &n);
    n = n * 2 - 1;

    while (n--) {
        scanf("%d", &i);
        s = s ^ i;
    }
    printf("%d\n", s);

    return 0;
}