#include "bits/stdc++.h"
using namespace std;
#define int long long
#define REP(i,n) for(int i = 0;i < (int)(n);++i)
#define RREP(i,n) for(int i = (int)n-1;i >= 0;--i)
#define FOR(i,s,n) for(int i = s;i < (int)n;++i)
#define RFOR(i,s,n) for(int i = (int)n-1;i >= s;--i)
#define ALL(a) a.begin(),a.end()
#define IN(a, x, b) (a<=x && x<b)
template<class T>inline void out(T t){cout<<t<<"\n";}
template<class T,class... Ts>inline void out(T t,Ts... ts){cout<<t<<" ";out(ts...);}
template<class T>inline bool CHMAX(T&a,T b){if(a<b){a = b;return true;}return false;}
template<class T>inline bool CHMIN(T&a,T b){if(a>b){a = b;return true;}return false;}
constexpr int INF = 1e18;

signed main(){
	int N,K;
	cin >> N >> K;
	if(K > N)out(-1);
	else out(K-1);
}