#include <bits/stdc++.h>

using namespace std;

#define rep(i, a, n) for (int i=(a); i < (n); i++)
#define per(i, a, n) for (int i=(n)-1; i >= (a); i--)
#define ll long long
#define x first
#define y second
const ll INF = 2e18;
typedef pair<ll,ll> pt;

int n;

	ll m;
	
ll go(vector<pt> &v){
	ll res = INF;
	vector<pair<pt, int>> xo;
	rep(i,0,n) xo.push_back({v[i], i});
	sort(xo.begin(), xo.end());
	set<pt> active;
	//for(auto a : xo) cout << a.x.x << " " << a.x.y << endl;
	//cout<<"---------"<<endl;
	rep(i,0,n){
		active.insert({xo[i].x.y, xo[i].x.x});
		if(i < n-1 && xo[i+1].x.x == xo[i].x.x){
			continue;
		}
		//cout << i << ": "<<yo[yi].x.y << ": ";
		//for(auto asdf : active) cout << asdf<<" ";cout<<endl;
		while(active.size() > (n / 2)){
			ll tr = active.begin()->first;
			while(active.size() && (active.begin()->first) == tr) {
				active.erase(active.begin());
			}
		}
		if(active.size() == n/2){
			//for(auto a : xs) cout << a << " "; cout << endl;
			ll cur = (xo[i].x.x + 1) * (m - active.begin()->first);
			//ll cur = (xo[i].x.x+1) * (m-yo[yi].x.y);
			//cout << "yes " << (*(--xs.end())) << " " << (*(--ys.end())) << ": " << cur << endl;
			res = min(res, cur);
		}
	}
	return res;
}

int main(void) {

	while(cin>>m>>n){
		vector<pair<ll,ll>> v(n);
		rep(i,0,n)cin>>v[i].x>>v[i].y;
		//cout<<n<< " "<<m<<"!!"<<endl;
		if(n%2) {
			cout<<-1<<endl;
			continue;
		}
		ll res = INF;
		rep(i,0,4){
			res = min(res, go(v));
			rep(j,0,n) {
				swap(v[j].x, v[j].y);
				v[j].y = m - 1 - v[j].y;
			}
		}
		if(res == INF) res = -1;
		cout << res << endl;
	}
	return 0;
}