#include <bits/stdc++.h>
using namespace std;
#define int long long
signed main(){
    int A,B;
    cin>>A>>B;
    if(gcd(A,B) != 1){
        cout<<"-1"<<endl;
        return 0;
    } 
    int ans = 0;
    for(int i=1;i<A*B;i++){
        bool ok = true;
        for(int j=0;j*A<=i;j++){
            if((i-j*A)%B == 0) ok = false;
        }
        if(ok) ans++;
    }
    cout<<ans<<endl;
}