#include <iostream>
#include <set>
#include <utility>
#include <algorithm>
#include <iterator>

using namespace std;

/*
long long f1(const multiset<long long> &mset, long long many) {
	long long high=1000000047LL, low=0;
	long long mid;
	
	if (mset.size()<many) return -1;
	if (mset.size()==many) return *(mset.rbegin());
	
	while (high>low+1) {
		mid=(high+low)/2;
		multiset<long long>::iterator it = mset.upper_bound(mid);
		
		if (distance(mset.begin(), it)==many) {
			multiset<long long>::iterator it2=mset.upper_bound(mid);
			it2--;
			return (*it2);
		}
		if (distance(mset.begin(), it) > many) high=mid;
		else low=mid;
	}
	return -1;
}


long long f1(multiset<long long> &mset, long long many) {	
	if (mset.size()<many) return -1;
	if (mset.size()==many) return *(mset.rbegin());
	
	while (mset.size()>many) {
		multiset<long long>::iterator it = mset.end();
		mset.erase(--it);
	}
	return *(mset.rbegin());
}

int main() {
	int n; 
	long long m;
	while (cin>>m>>n) {
		vector<pair<long long, long long> > input(n);
		multiset<long long> mset;
		
		for (int i=0; i<n; i++) cin>>input[i].first>>input[i].second;
		sort(input.begin(), input.end());
		
		int k=0;
		mset.insert(input[k++].second);
		
		long long best=-1;
		
		do {
			while (k<n && input[k].first==input[k-1].first) mset.insert(input[k++].second);
			long long high=f1(mset, n/2);
			
			if (high!=-1) {
				if (best==-1) best=(input[k-1].first+1)*(high+1);
				else best=min(best, (input[k-1].first+1)*(high+1));
			} 
			if (k<n) mset.insert(input[k++].second);
		} while (k<n);
		
		if (n%2==1 || best==-1) cout<<-1<<endl;
		else cout<<best<<endl;
	}
	
	return 0;
}
*/

int main() {
	int n; 
	long long m;
	while (cin>>m>>n) {
		vector<pair<long long, long long> > input(n);
		multiset<long long> mset;
		
		for (int i=0; i<n; i++) cin>>input[i].first>>input[i].second;
		sort(input.begin(), input.end());
		
		int k=0;
		long long best=-1, highest_del=-1;
		
		mset.insert(input[k++].second);	
		
		if (n%2==1) {
			cout<<-1<<endl;
			continue;
		}
		
		do {
			while (k<n && input[k].first==input[k-1].first) {
				long long tmp=input[k++].second;
				if (highest_del < tmp || highest_del==-1) mset.insert(tmp);
			}
			
			//mset
			/*
			cout<<"before"<<endl;
			for (multiset<long long>::iterator i=mset.begin(); i!=mset.end(); i++) cout<<*i<<" "; cout<<endl;
			*/
			
			while (mset.size()>n/2) {
				if (highest_del==-1) highest_del=*mset.rbegin();
				else highest_del=min(highest_del, *mset.rbegin());
				mset.erase(mset.lower_bound(*mset.rbegin()), mset.end());
			}
			//mset
			/*
			cout<<"after"<<endl;
			for (multiset<long long>::iterator i=mset.begin(); i!=mset.end(); i++) cout<<*i<<" "; cout<<endl;
			*/
			
			long long h=*mset.rbegin();
			if (mset.size()==n/2) {
				if (best==-1) best=(input[k-1].first+1)*(h+1);
				else best=min(best, (input[k-1].first+1)*(h+1));
			}
			
			if (k<n) {
				long long tmp=input[k++].second;
				if (tmp < highest_del || highest_del==-1) mset.insert(tmp);
			}
		} while (k<n);
		
		if (n%2==1 || best==-1) cout<<-1<<endl;
		else cout<<best<<endl;
	}
	
	return 0;
}