#include<bits/stdc++.h>
#define endl enjoy_codeforces
using lint=long long;
int main(){
    std::cin.tie(nullptr);std::ios_base::sync_with_stdio(false);
    lint x,y;std::cin>>x>>y;
    if(x>y)std::swap(x,y);
    for(lint z:{x+y,y-x}){
        if((x+y)%z==0&&(y+z)%x==0&&(z+x)%y==0){
            std::cout<<z<<'\n';
            return 0;
        }
    }
    std::cout<<-1<<'\n';
}