#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
#include <atcoder/all>


using namespace std;
using namespace atcoder;
#define rep(i,m,n,k) for (int i = (int)(m); i < (int)(n); i += (int)(k))
#define rrep(i,m,n,k) for (int i = (int)(m); i > (int)(n); i += (int)(k))
#define ll long long
#define list(T,A,N) vector<T> A(N);for(int i=0;i<(int)(N);i++){cin >> A[i];}
template<typename T> bool chmin(T& a, T b){if(a > b){a = b; return true;} return false;}
template<typename T> bool chmax(T& a, T b){if(a < b){a = b; return true;} return false;}
template<typename T> map<T,int> Counter(vector<T> X){map<T,int> C;for(auto x:X){C[x]++;}; return C;}
#define endl "\n"

int main(){
    int A,B;
    cin >> A >> B;
    int g;
    g = gcd(A,B);
    if(g!=1){
        cout << "-1" << endl;
        return 0;
    }
    set<int> S;
    int M = A*B;
    rep(i,0,M+1,1){
        S.insert(i);
    }
    rep(x,0,B+1,1){
        rep(y,0,A+1,1){
            if(A*x+B*y<=M){
                S.erase(A*x+B*y);
            }
        }
    }
    cout << S.size() << endl;
}