#include<bits/stdc++.h>
using namespace std;
#define int long long
#define pii pair<int,int>
const int N=2e3+10;
int n;bool v[N][N];
map<int,pii>mp;
bool solve()
{
	int a;scanf("%lld",&a);
	pii b=mp[a];
	int x=b.first,y=b.second;
	v[x][y]=1;
	bool bk=1;
	for(int j=1;j<=n;j++)
		if(!v[x][j])bk=0;
	if(bk)return 1;
	bk=1;
	for(int i=1;i<=n;i++)
		if(!v[i][y])bk=0;
	if(bk)return 1;
	if(x==y)
	{
		bk=1;
		for(int i=1;i<=n;i++)
			if(!v[i][i])bk=0;
		if(bk)return 1;
	}
	if(y==n-x+1)
	{
		bk=1;
		for(int i=1;i<=n;i++)
			if(!v[i][n-i+1])bk=0;
		if(bk)return 1;
	}
	return 0;
}
signed main()
{
	memset(v,0,sizeof v);
	int T;scanf("%lld%lld",&n,&T);
	for(int i=1;i<=n;i++)for(int j=1;j<=n;j++)
		mp[n*(i-1)+j]={i,j};
	for(int i=1;i<=T;i++)
		if(solve()){printf("%lld\n",i);return 0;}
	puts("-1");return 0;
}