#define DEBUG 1
#include <bits/stdc++.h>
#define loop(n) for (lint ngtkana_is_a_genius = 0; ngtkana_is_a_genius < lint(n); ngtkana_is_a_genius++)
#define rep(i, begin, end) for (lint i = lint(begin); (i) < lint(end); i++)
#define all(v) v.begin(), v.end()
#define rand(l, r) std::uniform_int_distribution<>(l, r)(mt)
using lint = long long;
auto mt = std::mt19937_64(std::random_device{}());
auto cmn = [](auto&& a, auto b){ if (a > b) {a = b; return true;} return false; };
auto cmx = [](auto&& a, auto b){ if (a < b) {a = b; return true;} return false; };
void debug_impl() { std::cerr << std::endl; }
template <typename Head, typename... Tail>
void debug_impl(Head head, Tail... tail) { std::cerr << " " << head; debug_impl(tail...); }
#if DEBUG
#define debug(...)\
    do {\
        std::cerr << std::boolalpha << "[" << #__VA_ARGS__ << "]:";\
        debug_impl(__VA_ARGS__);\
        std::cerr << std::noboolalpha;\
    } while (false)
#else
#define debug(...) {}
#endif

int main() {
    std::cin.tie(0); std::cin.sync_with_stdio(false);
    int A, B; std::cin >> A >> B;
    if(std::__gcd(A,B)!=1){
        std::cout << -1 << std::endl;
        exit(0);
    }
    std::vector<int>ckd(A*B,false);
    for(int i=0;i<A;i++){
        for(int j=0;j<B;j++){
            if(i*B+j*A<A*B){
                ckd.at(i*B+j*A)=true;
            }
        }
    }
    int ans=0;
    for(int i=0;i<A*B;i++){
        ans+=!ckd.at(i);
    }
    std::cout << ans << std::endl;
    return 0;
}