#include <bits/stdc++.h>

using namespace std;

using ll = long long;

int dfs(int cur, int par, vector< vector<int> >& g) {
    int cnt = 0;
    for (int nex : g[cur]) {
        if (nex == par) continue;
        cnt += 1 + dfs(nex, cur, g);
    }
    return cnt;
}

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int n, K;
    cin >> n >> K;
    if (K > n) {
        cout << -1 << "\n";
        return 0;
    } else {
        cout << K - 1 << "\n";
    }
    /*
    vector< vector<int> > g(n);
    for (int i = 1; i < n; ++i) {
        int a, b;
        cin >> a >> b;
        --a; --b;
        g[a].push_back(b);
        g[b].push_back(a);
    }
    cout << dfs(0, -1, g) << "\n";*/
    return 0;
}