#include <bits/stdc++.h>
using namespace std;
using llong = long long int;
const static int MOD = 1000000007;
const static int IINF = INT_MAX;
const static llong LINF = LLONG_MAX;
const static int dx[4] = {1, 0, -1, 0};
const static int dy[4] = {0, 1, 0, -1};
template <typename T>
struct UF {
  vector<int> par;
  vector<T> sz;
  UF(int n) {
    par.resize(n);
    sz.resize(n);
    for (int i = 0; i < n; ++i) {
      par[i] = i;
      sz[i] = 1;
    }
  }
  int find_root(int x) { return par[x] == x ? x : par[x] = find_root(par[x]); }
  bool same(int x, int y) { return find_root(x) == find_root(y); }
  void unite(int x, int y) {
    x = find_root(x);
    y = find_root(y);
    if (x == y) return;
    if (sz[x] < sz[y]) swap(x, y);
    sz[x] += sz[y];
    par[y] = x;
  }
  T size_of_tree(int x) { return sz[find_root(x)]; }
};
llong my_gcd(llong a, llong b) { return (a ? my_gcd(b % a, a) : b); }
llong my_lcm(llong a, llong b) { return a * b / my_gcd(a, b); }
llong facmod(llong n) {
  if (n == 0) return 1;
  llong res = n;
  for (int i = n - 1; i > 0; --i) {
    res = (res * i) % MOD;
  }
  return res;
}
llong powmod(llong a, llong n) {
  llong res = 1;
  while (n > 0) {
    if (n & 1) res = (res * a) % MOD;
    a = (a * a) % MOD;
    n >>= 1;
  }
  return res;
}
llong modinv(llong n) { return powmod(n, MOD - 2); }
llong combmod(llong n, llong k) {
  llong res = 1;
  for (int i = 0; i < k; ++i) {
    res = (res * (n - i)) % MOD;
  }
  res = (res * modinv(facmod(k))) % MOD;
  return res;
}
int main(int argc, char *argv[]) {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int n, m;
  cin >> n >> m;
  if (m < n) {
    cout << -1 << endl;
    return 0;
  }
  cout << n << endl;
  return 0;
}
