#include<bits/stdc++.h>
   
using namespace std;
  
#define rep(i,n) for(int i=0;i<(n);++i)
#define reps(i,n) for(int i=1;i<=(n);++i)
#define all(x) (x).begin(),(x).end()
#define Fixed fixed << setprecision(14)
#define int int64_t
using pii = pair<int,int>;
constexpr int INF  = 0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr int mod1 = 1e9+7; 
constexpr int mod2 = 998244353;
 
template <class A, class B> inline bool chmax(A &a, const B &b) { return b > a && (a = b, true); }
template <class A, class B> inline bool chmin(A &a, const B &b) { return b < a && (a = b, true); }
 
template <class T> using min_heap = priority_queue<T,vector<T>,greater<T> >;
template <class T> using max_heap = priority_queue<T>;
template <class A,class B> using umap = unordered_map<A,B>;
   
int gcd(int a,int b){ return b ? gcd(b,a % b) : a;}
int lcm(int a,int b){ return a / gcd(a,b) * b;}

signed main(void){
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);

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

  if(n < k) return cout << "-1\n",0;
  else cout << k-1 << '\n';

  return 0;
}