#include <bits/stdc++.h>
using namespace std;

using ll = long long;
using pii = pair<int, int>;
using vi = vector<int>;
using vvi = vector<vi>;
using vd = vector<double>;
using vc = vector<char>;
using vs = vector<string>;
using vll = vector<ll>;
using vpii = vector<pii>;
using mpii = map<int, int>;
using mpci = map<char, int>;
using mpsi = map<string, int>;
using si = set<int>;
using sll = set<ll>;

#define rep(i, n) for(int(i)=0;(i)<(n);(i)++)
const int MAX = 510000;
const int MOD = 1000000007;

long long fac[MAX], finv[MAX], inv[MAX];

// テーブルを作る前処理
void COMinit() {
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for (int i = 2; i < MAX; i++){
        fac[i] = fac[i - 1] * i % MOD;
        inv[i] = MOD - inv[MOD%i] * (MOD / i) % MOD;
        finv[i] = finv[i - 1] * inv[i] % MOD;
    }
}

// 二項係数計算
long long COM(int n, int k){
    if (n < k) return 0;
    if (n < 0 || k < 0) return 0;
    return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

// a^n mod を計算する
long long modpow(long long a, long long n, long long mod) {
    long long res = 1;
    while (n > 0) {
        if (n & 1) res = res * a % mod;
        a = a * a % mod;
        n >>= 1;
    }
    return res;
}

bool custom1 (pii a, pii b) {
  return (a.first < b.first);
}

bool custom2 (pii a, pii b) {
  return (a.second < b.second);
}

long long gcd(long long a, long long b) {
  if(a % b == 0) return b;
  else return gcd(b, a % b);
}

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

  int LCM = a * b / gcd(a, b);

  if(gcd(a, b) != 1) {
    cout << -1 << endl;
  }
  else {
    if(a == 1 || b == 1) cout << 0 << endl;
    else {
      vector<int> v(LCM+1);
      for(int i=0;i<v.size();i++) {
        if(i % a == 0) v[i] = 1;
      }
      for(int i=0;i<v.size();i++) {
        if(v[i] == 1) {
          for(int j=1;i+b*j<=LCM;j++) {
            v[i+b*j] = 1;
          }
        }
      }

      int ans = 0;

      for(int i=0;i<v.size();i++) {
        if(v[i] == 1) continue;
        else ans++;
      }

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