#include <iostream>
#include <string_view>
#include <algorithm>
#include <ranges>
using ll = long long;
#define ALL(x) (std::ranges::begin(x)),(std::ranges::end(x))

int main() {
    using namespace std::literals;
    ll a[3];
    std::cin >> a[0] >> a[0];
    if(char c;std::cin.get(c), c == ' ') {
        std::cout << "\"assert\"\n"sv;
        return 0;
    }
    std::cin >> a[1] >> a[2];
    ll sum = a[0] + a[1] + a[2];
    a[0] = sum - a[0];
    a[1] = sum - a[1];
    a[2] = sum - a[2];
    std::sort(ALL(a));
    std::cout << (a[1] == a[2] ? a[0] : a[1]) << '\n';
}