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

const int maxn=100000000;
int n,m,ans,x,a[maxn];
bool f;

int main()
{
	freopen("leader.in","r",stdin);
	freopen("leader.out","w",stdout);
	
	cin>>n>>m;
	
	for (int i=1; i<=m; i++)
	{
		cin>>x;
		a[x]++;
	}
	for (int i=1; i<=n; i++)
	{
		if(a[i]!=0 && a[i]>m/2)
		{
			f=true;
			cout<<i<<endl;
			break;
		}
	}
	if(!f)
	{
		cout<<-1<<endl;
	}
	return 0;
}