#include <bits/stdc++.h>

using namespace std;

#define rep(i, n) for(int i = 0; i < static_cast<int>(n); ++i)
#define per(i, n) for(int i = static_cast<int>(n)-1; i >= 0; --i)
#define eee(a) (cerr << (#a) << ": " << (a) << endl);
#define ppp(a) (cout << (a) << endl); 
#define cop(a) for(auto x:(a)){cout << x << endl;}

using ll = long long;

constexpr int dx[4] = {1, 0, -1, 0};
constexpr int dy[4] = {0, 1, 0, -1};

constexpr char sp = ' ';
constexpr ll MOD = 1e9+7;
constexpr ll Mod = 998244353;
constexpr int INF = 1e9;
constexpr ll LINF = 1e18;
constexpr double phi = (1+sqrt(5))/2;
constexpr double pi = 3.14159265358979323846264338327950288;
inline ll gcd(ll a, ll b){return b?gcd(b,a%b):a;}
inline ll lcm(ll a, ll b){return a/gcd(a,b)*b;}
inline ll inverse(ll n, ll m=MOD){ll a=n,b=m,x=1,y=0;while(b){ll t=a/b;a-=t*b;swap(a,b);x-=t*y;swap(x,y);}return (a%m+m)%m;}
inline void Yes(){puts("Yes");}
inline void No(){puts("No");}
inline void Judge(bool b){b?Yes():No();}
inline void YES(){puts("YES");}
inline void NO(){puts("NO");}
inline void JUDGE(bool b){b?YES():NO();}
inline ll powmod(ll b, ll e, ll m=MOD){ll r=1;while(e>0){if(e&1)r=(r%m)*(b%m)%m;b=(b%m)*(b%m)%m;e>>=1;}return r;}
inline double distance(ll x1, ll y1, ll x2, ll y2){return sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));}
inline double d2r(double d){return d*pi/180;}
inline double r2d(double r){return r*180/pi;}

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
template<typename T1, typename T2> ostream& operator<<(ostream& s, const pair<T1, T2>& p) {return s << "(" << p.first << ", " << p.second << ")";}
template<typename T> ostream& operator<<(ostream& s, const vector<T>& v) {
    s<<'[';for(size_t i=0;i<v.size();++i){s << v[i];if(i < v.size()-1) s << ", ";}s<<']';return s;
}

int main(){
    cin.tie(0); ios::sync_with_stdio(0); cout<<fixed<<setprecision(15);
    
    int n, m;
    cin >> n >> m;

    if(n <= m){
        cout << 1 << '\n';
        return 0;
    }
    if(n%2 or n%2 == 0 and n > 2*m){
        cout << -1 << '\n';
        return 0;
    }

    cout << 2 << '\n';
    return 0;
}