#include <iostream>
#include <cstdio>

using namespace std;

int main() {
    #ifndef ONLINE_JUDGE
    freopen("input.txt","r",stdin);
    #endif // ONLINE_JUDGE

    long long a, b;

    cin >> a >> b;

    while(b >= a && b % 2 == a % 2) {
        --b;
    }

    if(b < a) {
        cout << -1 << '\n';
        return 0;
    }

    cout << b - a + 1 << '\n';

    return 0;
}