#include"bits/stdc++.h"
using namespace std;
#define REP(k,m,n) for(int (k)=(m);(k)<(n);(k)++)
#define rep(i,n) REP((i),0,(n))
using ll = long long;

int main()
{
    ll N, K;
    cin >> N >> K;
    cout << (N < K ? -1 : K - 1) << endl;
    return 0;
}