#include using namespace std; struct cww{cww(){ios::sync_with_stdio(false);cin.tie(0);}}star; #define all(c) (c).begin(), (c).end() #define rall(c) (c).rbegin(), (c).rend() #define vv(type, c, m, n, i) vector> c(m, vector(n, i)) #define rep(i,a,n) for(int i=(a), i##_len=(n); ii##_len; --i) #define len(x) ((int)(x).size()) #define mp make_pair #define eb emplace_back #define em emplace #define fi first #define se second #define get(a, i) get(a) #define mt make_tuple template inline T min(vector& v) {return *min_element(v.begin(), v.end());} template inline T max(vector& v) {return *max_element(v.begin(), v.end());} #define sum(a, b) accumulate(a, b) #define chmax(a,b) if(ab)a=b #define print0(v) for(auto i : v) { cout << i << " "; } cout << endl; template inline void print(T t){cout << t << "\n";} template inline void print(H h, T... t){cout << h << " ";print(t...);} typedef long long ll; typedef long double ld; typedef vector vi; typedef vector vd; typedef vector vll; typedef vector vs; typedef vector vb; typedef pair pii; int solve() { int n, m; cin >> n >> m; if (n & 1 && n != m) return -1; if (n <= m) return 1; if (n <= m * 2) return 2; return -1; } int main() { print(solve()); return 0; }