#include "bits/stdc++.h"
using namespace std;
#define int long long
#define FOR(i, a, b) for(int i=(a);i<(b);i++)
#define RFOR(i, a, b) for(int i=(b-1);i>=(a);i--)
#define REP(i, n) for(int i=0; i<(n); i++)
#define RREP(i, n) for(int i=(n-1); i>=0; i--)
#define REP1(i, n) for(int i=1; i<=(n); i++)
#define RREP1(i, n) for(int i=(n); i>=1; i--)
#define ALL(a) (a).begin(),(a).end()
#define UNIQUE_SORT(l) sort(ALL(l)); l.erase(unique(ALL(l)), l.end());
#define CONTAIN(a, b) find(ALL(a), (b)) != (a).end()
#define out(...) printf(__VA_ARGS__)

int dxy[] = {0, 1, 0, -1, 0};

void solve();
signed main()
{
#if DEBUG
    std::ifstream in("input.txt");
    std::cin.rdbuf(in.rdbuf());
#endif
    cin.tie(0);
    ios::sync_with_stdio(false);
    solve();
    return 0;
}

/*================================*/
#if DEBUG
#define SIZE 100
#else
#define SIZE 123450
#endif

int A,B;

int check(int C) {
    if (A==B || B==C || C==A) return false;
    if ((A+B)%C!=0) return false;
    if ((B+C)%A!=0) return false;
    if ((C+A)%B!=0) return false;
    return true;
}

int ans() {
    int AB = A+B;
    REP1(n, sqrt(AB)+1) {
        if (AB%n==0) {
            if (check(n)) return n;
            if (check(AB/n)) return AB/n;
        }
    }
    return -1;
}

void solve() {
    cin>>A>>B;
    
    cout << ans() << endl;
}