n, m = map(int,input().split(" "))
correct = list(map(int, input().split(" ")))
wrong = list(map(int, input().split(" ")))
min_possible_v = 2*(min(correct))
min_correct = max(correct)
max_possible = min(wrong)
if min_correct >= max_possible:
print(-1)
else:
if min_possible_v >= max_possible:
print(-1)
else:
print(max(min_possible_v, min_correct))
#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define N 100001
ll prime[N];
vector<int>all_primes;
ll prime_factors[N];
void seive()
{
memset(prime,0,sizeof(prime));
prime[0]=1;prime[1]=1;
for(ll i=2;i*i<=N-1;i++)
{
if(prime[i]==0)
{
for(ll j=i*i;j<N;j+=i)
{
prime[j]=1;
}
}
}
for(int i=2;i<N;i++)
{
if(prime[i]==0)
all_primes.push_back(i);
}
}
void spf()
{
for(ll i=1;i<N;i++)
{
prime_factors[i]=i;
}
for(ll i=2;i*i<=N-1;i++)
{
for(ll j=i*i;j<=N-1;j+=i)
{
if(prime_factors[j]==j)
prime_factors[j]=i;
}
}
}
int main()
{
int n,m;
cin>>n>>m;
int a[n],b[m];
for(int i=0;i<n;i++)
{
cin>>a[i];
}
for(int i=0;i<m;i++)
{
cin>>b[i];
}
sort(a,a+n);
sort(b,b+m);
int ans=-1;
if(n==1 && (a[0]*2)<b[0])
ans=a[0]*2;
else if(a[n-1]<b[0] && a[n-1]>=(2*a[0]))
ans=a[n-1];
else if((a[0]*2)>=a[n-1] && (a[0]*2)<b[0])
ans=a[0]*2;
cout<<ans;
}
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