#include <bits/stdc++.h>

#define FOR(i,a,b) for (int i = (a); i <= (b); i++)
#define FORD(i,a,b) for (int i = (a); i >= (b); i--)
#define REP(i,b) for (int i = 0; i < (b); i++)

using namespace std;

typedef long long ll;

void vloz(map<int, set<int> > & d, int a, int b){
	if(d.find(a) == d.end()) d[a] = set<int>();
	d[a].insert(b);
}

ll solve(const vector<int>& xs, const vector<int>& ys, int m, int n){
	map<int, set<int> > stlpce;
	REP(i,n){
		vloz(stlpce, xs[i], ys[i]);
	}
	ll h = m;
	map<int, set<int> >::iterator st_it = stlpce.begin();
	priority_queue<int> q;
	ll res = -1;
	while(st_it != stlpce.end()){
		ll r = st_it->first;
		set<int>::iterator add_it = st_it->second.begin();
		while(add_it!=st_it->second.end() && *add_it <= h){
			q.push(*add_it);
			add_it++;
		}
		while(q.size() > n/2){
			h = q.top();
			while(!q.empty() && q.top() == h) q.pop();
			h--;
		}
		if (q.size() == n/2){
			ll h_t = q.top();
			//cout<<(h_t+1)<< " "<<(r+1)<<endl;
			if(res > (h_t+1) * (r+1) || res == -1) {
				res = (h_t+1) * (r+1);
			}
		}
		st_it++;
	}
	return res;
}

int main() {
	int n, m;
	while (scanf("%d", &m) == 1) {
		scanf("%d", &n);
		vector<int> xs (n);
		vector<int> ys (n);
		REP(i,n) scanf("%d%d", &xs[i], &ys[i]);
		if(n%2 == 1){
			printf("%d\n", -1);
			continue;
		}
		ll res = solve(xs, ys, m, n);
//cout<<"#"<<endl;
		REP(i,n){
			xs[i] = m - xs[i] - 1;
		}
		ll temp = solve(xs,ys,m,n);
		if(temp != -1){
			res = min(temp, res);
		}
		//cout<<"#"<<endl;
		REP(i,n){
			ys[i] = m - ys[i] - 1;
		}
		temp = solve(xs,ys,m,n);
		if(temp != -1){
			res = min(temp, res);
		}
		
		//cout<<"#"<<endl;
		
		REP(i,n){
			xs[i] = m - xs[i] - 1;
		}
		temp = solve(xs,ys,m,n);
		if(temp != -1){
			res = min(temp, res);
		}
		//cout<<"#"<<endl;
		printf("%lld\n", res);
	}
	return 0;
}