#include<bits/stdc++.h>
using namespace std;
int main()
{
    int A, B, C;
    cin >> A >> B;
    if (A != B)
    {
        C = (A + B) / 2;
    }
    if ((A + C) == 2 * B && (B + C) == 2 * A && A != C && B != C)
    {
        cout << C << endl;
    }
    else
    {
        cout << -1 << endl;
    }
    return 0;
}