#include <bits/stdc++.h> using namespace std::literals::string_literals; using i64 = std::int_fast64_t; using std::cout; using std::cerr; using std::endl; using std::cin; template<typename T> std::vector<T> make_v(size_t a){return std::vector<T>(a);} template<typename T,typename... Ts> auto make_v(size_t a,Ts... ts){ return std::vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...)); } int main() { int a, b; scanf("%d%d", &a, &b); if(std::__gcd(a, b) != 1) printf("-1\n"); else { std::vector<bool> cnt(2 * a * b + 1, false); for(int x = 0; x <= b; x++) { for(int y = 0; y <= a; y++) { cnt[a * x + b * y] = true; } } int ans = 0; for(auto v: cnt) ans += !v; printf("%d\n", ans >> 1); } return 0; }