#include <cstdio>
#include <iostream>
#include <map>
using namespace std;
map<unsigned long long int, int> M;
int main(void) {
int n;
while (scanf("%d", &n) != EOF) {
M.clear();
while (n--) {
unsigned long long int tmp;
scanf("%llu", &tmp);
if (M.find(tmp) == M.end()) {
M[tmp] = 1;
} else ++M[tmp];
}
for (map<unsigned long long int, int>::iterator itor = M.begin(); itor != M.end(); ++itor) {
if (itor->second != 3) {
printf("%llu\n", itor->first);
break;
}
}
}
return 0;
}
