#include <bits/stdc++.h>

#define rep(X,N) for(ll X = 0; X < (N); X++)
#define PI (acos(-1.0))
#define pback push_back
#define mpair make_pair
#define MODN 1000000007
#define ALL(V) (V).begin(),(V).end()
#define CERR if(false) cerr
#define INT_MAX_HALF (INT_MAX / 2)
#define EPS (1e-10)

using namespace std;

typedef long long ll;

int main(){

    int n,k;
    cin >> n >> k;

    int ans = k - 1;

    if(k > n) ans = -1;

    cout << ans << endl;



    
    return 0;
}