#include <iostream>
#include <cstdio>
#include <cstring>
#include <climits>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <algorithm>
#include <set>
#include <deque>
#include <queue>
#include <string>
#include <map>
#include <stack>
#include <sstream>
#include <vector> 
#include <ctime>
#include <bitset>
using namespace std;


int n,m;
bool b;
map<int,int> a;

int main()
{
	freopen("leader.in","r",stdin);
	freopen("leader.out","w",stdout); 
	
	scanf("%d%d",&n,&m);
	for(int i=1;i<=m;i++)
	{
		int x; cin>>x;
		a[x]++;
	}
	int p=m/2;
	for(map<int,int>::iterator it=a.begin();it!=a.end();it++)
	{
//		int q=*it;
		if(it->second>p)
		{
			b=true;
			printf("%d",*it);
			break;
		}
	}
	if(b==false)
	{
		cout<<"-1"<<endl;
	}
	return 0;
}