#include <bits/stdc++.h>
#define forr(i,a,b) for(int i=(a);i<(b);i++)
#define forn(i,n) forr(i,0,n)
#define dforn(i,n) for(int i=n-1;i>=0;i--)
#define forall(it,v) for(auto it=v.begin();it!=v.end();it++)
#define sz(c) ((int)c.size())
#define rsz resize
#define pb push_back
#define mp make_pair
#define lb lower_bound
#define ub upper_bound
#define fst first
#define snd second

#ifdef ANARAP
//local
#else
//judge
#endif

using namespace std;

typedef long long ll;
typedef pair<int,int> ii;

#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
//<key,mapped type,comparator,...>
typedef tree<ii,null_type,less<ii>,rb_tree_tag,
	tree_order_statistics_node_update> indexed_set;
//find_by_order(i) devuelve iterador al i-esimo elemento
//order_of_key(k): devuelve la pos del lower bound de k
//Ej: 12, 100, 505, 1000, 10000.
//order_of_key(10) == 0, order_of_key(100) == 1,
//order_of_key(707) == 3, order_of_key(9999999) == 5

const ll INF = 2e18;

ll solve(vector<ii>& v, int len)
{
	sort(v.begin(), v.end());
	indexed_set s;
	int pos = 0;
	ll ret = INF;
	while(pos < sz(v))
	{
		int from = pos;
		while(pos < sz(v) && v[pos].fst == v[from].fst) s.insert(mp(v[pos++].snd, sz(s)));
		if(sz(s) < sz(v)/2) continue;
		auto ite = s.find_by_order(sz(v)/2-1);
		if(sz(s) > sz(v)/2 && next(ite)->fst == ite->fst) continue;
		ret = min(ret, (1LL+v[from].fst) * (1LL+ite->fst));
	}
	return ret;
}

int main()
{
	// agregar g++ -DANARAP en compilacion
	#ifdef ANARAP
		freopen("input.in", "r", stdin);
		//freopen("output","w", stdout);
	#endif
	ios::sync_with_stdio(false);
	cin.tie(NULL);
	cout.tie(NULL);
	int len,n;
	while(cin >> len >> n)
	{
		vector<ii> v(n);
		forn(i,n) cin >> v[i].fst >> v[i].snd;
		if(n%2)
		{
			cout << "-1\n";
			continue;
		}
		ll ans = solve(v, len);
		forn(i,n) v[i].fst = len-1-v[i].fst;
		ans = min(ans, solve(v, len));
		forn(i,n) v[i].snd = len-1-v[i].snd;
		ans = min(ans, solve(v, len));
		forn(i,n) v[i].fst = len-1-v[i].fst;
		ans = min(ans, solve(v, len));
		if(ans == INF) ans = -1;
		cout << ans << '\n';
	}
	return 0;
}