#include "bits/stdc++.h" using namespace std; #define FOR(i,j,k) for(int (i)=(j);(i)<(int)(k);++(i)) #define rep(i,j) FOR(i,0,j) #define each(x,y) for(auto &(x):(y)) #define mp make_pair #define mt make_tuple #define all(x) (x).begin(),(x).end() #define debug(x) cout<<#x<<": "<<(x)<<endl #define smax(x,y) (x)=max((x),(y)) #define smin(x,y) (x)=min((x),(y)) #define MEM(x,y) memset((x),(y),sizeof (x)) #define sz(x) (int)(x).size() #define pb push_back typedef long long ll; typedef pair<int, int> pii; typedef vector<int> vi; typedef vector<ll> vll; int main(){ ios::sync_with_stdio(false); cin.tie(0); int n, m; cin >> n >> m; if (n <= m)cout << 1 << endl; else if (n % 2 == 0 && n / 2 <= m) { cout << 2 << endl; } else { cout << -1 << endl; } }