#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("Yes");}else{puts("No");}
#define MAX_N 200005

// long version
long long int GCD(long long int x, long long int y){
    if(x>y){
        long long int swap = x;
        x = y;
        y = swap;
    }
    while(true){
        if(y%x==0){
            return x;
        }else{
            long long int amari = y%x;
            y = x;
            x = amari;
        }
    }
}

int main() {
    int a, b;
    cin >> a >> b;

    if(GCD(a,b) > 1){
        cout << -1 << endl;
        return 0;
    }

    int dp[a * b + 1] = {};

    rep(i,b+1){
        int j = i * a;
        while(j <= a * b){
            dp[j] = 1;
            j += b;
        }
    }

    int ans = 0;

    rep(i,a*b+1)if(dp[i] == 0)ans++;

    cout << ans << endl;
    return 0;
}