#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <climits>
using namespace std;

const int maxn=10000+10;
int a[maxn];
int n,m;
bool f=false;

int main()
{
	freopen("leader.in","r",stdin);
	freopen("leader.out","w",stdout);
	
	scanf("%d%d",&n,&m);
	int t;
	for(int i=1;i<=m;i++)
	{
		scanf("%d",&t);
		a[t]++;
	}
	m>>=1;
	for(int i=1;i<=n;i++)
	{
		if(a[i]>m)
		{
			f=true;
			printf("%d\n",i);
			break;
		}
	}
	if(!f)
	{
		printf("-1\n");
	}
	return 0;
}