#include <iostream>
#include <string>
#include <vector>
#include <queue>
#include <deque>
#include <algorithm>
#include <set>
#include <map>
#include <bitset>
#include <cmath>
#include <functional>
#include <iomanip>
#define vll vector<ll>
#define vvvl vector<vvl>
#define vvl vector<vector<ll>>
#define VV(a, b, c, d) vector<vector<d>>(a, vector<d>(b, c))
#define VVV(a, b, c, d) vector<vvl>(a, vvl(b, vll (c, d)));
#define re(c, b) for(ll c=0;c<b;c++)
#define rep(a,b,c) for(ll a=b;a<c;a++)
#define all(obj) (obj).begin(), (obj).end()
typedef long long int ll;
typedef long double ld;
using namespace std;

void get(vll &a){re(i,a.size()) scanf("%lld",&a[i]);}
void get(vvl &a){re(i,a.size()) re(j,a[0].size()) scanf("%lld",&a[i][j]);}
void print(vll &a){re(i,a.size()) cout<<a[i]<<(i==a.size()-1?"\n":" ");}
void print(vvl &a){re(i,a.size()) re(j,a[0].size())cout<<a[i][j]<<(j==a[0].size()-1?"\n":" ");}

ll gcd(ll a, ll b){
  if(b<a) swap(a, b);
  ll r = a % b;
  if(r==0) return b;
  while(r!=0) r = a % b, a = b, b = r;
  return a;
}
ll lcm(ll a, ll b){return (a*b)/gcd(a, b);}
vll extgcd(ll a, ll b) {
  ll x, y;
  for (ll u = y = 1, v = x = 0; a;) {
    ll q = b / a;
    swap(x -= q * u, u);
    swap(y -= q * v, v);
    swap(b -= q * a, a);
  }
  return vll{x, y, b};
}
int main(int argc, char const *argv[]) {
  ll a, b;std::cin >> a >> b;
  ll sa = max(a, b) - min(a, b);
  ll ad = a + b;
  if((a+b)%sa==0&&sa!=a&&sa!=b&&(sa+a)%b==0&&(sa+b)%a==0) std::cout << sa << '\n';
  else if((ad+b)%a==0&&(ad+a)%b==0) std::cout << ad << '\n';
  else std::cout << -1 << '\n';
  return 0;
}