#include #include #include #include #include #include #include #include #include #include #include #include using namespace std; #define REP(i, n) for(ll i = 0;i < n;i++) #define REPR(i, n) for(ll i = n;i >= 0;i--) #define FOR(i, m, n) for(ll i = m;i < n;i++) #define FORR(i, m, n) for(ll i = m;i >= n;i--) #define REPO(i, n) for(ll i = 1;i <= n;i++) #define ll long long #define INF (ll)1 << 60 #define MINF (-1 * INF) #define ALL(n) n.begin(),n.end() #define MOD 1000000007 #define P pair int main() { ll a, b; cin >> a >> b; ll c = abs(a - b); if ((a + c) % b == 0 and (b + c) % a == 0 and (b + a) % c == 0) { cout << c << endl; return 0; } c = a + b; if ((a + c) % b == 0 and (b + c) % a == 0 and (b + a) % c == 0) { cout << c << endl; return 0; } c = min(a, b) - abs(a - b); if (c > 0) { if ((a + c) % b == 0 and (b + c) % a == 0 and (b + a) % c == 0) { cout << c << endl; return 0; } } cout << -1 << endl; }