#include <bits/stdc++.h>

using namespace std;
using ll = long long;
using PII = std::pair<int, int>;
using PLL = std::pair<ll, ll>;

#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep2(i, s, n) for (int i = (s); i < (int)(n); i++)


const int mod = 1e9 + 7;

ll cal(ll r, ll c, char op)
{
    if (op == '*')
        return r * c;
    if (op == '+')
        return r + c;
    return 0;
}

int main()
{

#ifdef DEBUG
    cout << "DEBUG MODE" << endl;
    ifstream in("input.txt"); //for debug
    cin.rdbuf(in.rdbuf());    //for debug
#endif

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

    if (n < k)
        ans = -1;
    else
        ans = k - 1;

    cout << ans << endl;
    return 0;
}