#include <bits/stdc++.h>

using namespace std;

int main() {
    vector<int> v1;
    while (true) {
        int c;
        cin >> c;
        if (c == 0)break;
        v1.push_back(c);
    }
    if (v1.size() % 2 == 1) cout << v1[v1.size() / 2] << endl;
    else cout << v1[v1.size() / 2 - 1] + v1[v1.size() / 2] << endl;
    return 0;
}