#include<bits/stdc++.h>
#define rep(i, n) for(int i=0;i<(n);++i)
#define per(i, n) for(int i=(n)-1;i>=0;--i)
#define repa(i, n) for(int i=1;i<(n);++i)
#define foreach(i, n) for(auto &i:(n))
#define pii pair<int, int>
#define pll pair<long long, long long>
#define all(x) (x).begin(), (x).end()
#define bit(x) (1ll << (x))
using ll = long long;
//const ll MOD = (ll)1e9+7;
const ll MOD = 998244353;
const int INF = (ll)1e9+7;
const ll INFLL = (ll)1e18;
using namespace std;
template<class t>
using vvector = vector<vector<t>>;
template<class t>
using vvvector = vector<vector<vector<t>>>;
template<class t>
using priority_queuer = priority_queue<t, vector<t>, greater<t>>;
template<class t, class u> bool chmax(t &a, u b){if(a<b){a=b;return true;}return false;}
template<class t, class u> bool chmin(t &a, u b){if(a>b){a=b;return true;}return false;}
#ifdef DEBUG
#define OUTPUT(x) (output(x), output("\n"))
#else
#define OUTPUT(x) (void)0
#endif

ll modpow(ll x, ll b){
    ll res = 1;
    while(b){
        if(b&1)res = res * x % MOD;
        x = x * x % MOD;
        b>>=1;
    }
    return res;
}

ll modinv(ll x){
    return modpow(x, MOD-2);
}

bool was_output = false;
template<class t>
void output(t a){
    if(was_output)cout << " ";
    cout << a;
    was_output = true;
}
void outendl(){
    was_output = false;
    cout << endl;
}


ll gcd(ll a, ll b){
    while(b){
        a %= b;
        swap(a, b);
    }
    return a;
}

ll lcm(ll a, ll b){
    return a / gcd(a, b) * b;
}

int main(){
    ll a;
    ll b;
    cin >> a >> b;
    ll lc = lcm(a, b);
    if(gcd(a, b)!=1){
        cout << -1 << endl;
        return 0;
    }
    cout << (lc - lc / a - lc / b + 1) / 2 << endl;
    return 0;
}