#include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;
const int maxn=2000000+10;
int c[maxn],f[maxn];
int a[maxn],b[maxn];
long int n;
int m;
int q,p;
int main()
{
	freopen ("leader.in","r",stdin);
	freopen ("leader.out","w",stdout);
	cin>>n>>m;
	for(int i=1;i<=m;i++)
	{
		cin>>a[i];
	}
	sort(a+1,a+m+1);
	int l=0;int ans=1;
	for(int i=1;i<=m;i++)
	{	ans=1;
		if(a[i]>=a[i+1] ) ans++;
		if(a[i]<a[i+1] || i==m)
		{
			if(ans>l) 
			{
				l=ans;p=a[i];
			}
		}
	}
			
	if(l>=m/2) cout<<p<<endl;
	else cout<<"-1"<<endl;
	return 0;
}