#include <iostream>
#include <stdint.h>
#include <vector>
using namespace std;

struct STree {
    int64_t x, y;
};
vector<STree> trees;
int64_t side;

bool roh(bool x1, bool y1, int64_t sx, int64_t sy) {
    size_t in = 0;
    for (size_t i = 0; i < trees.size(); ++i) {
	if (x1) {
	    if (trees[i].x < side - sx)
		continue;
	} else {
	    if (trees[i].x >= sx)
		continue;
	}
	if (y1) {
	    if (trees[i].y < side - sy)
		continue;
	} else {
	    if (trees[i].y >= sy)
		continue;
	}
	in++;
    }
    //std::cout << "Trees in: " << in << " for sx: " << sx << " sy: " << sy << " area: " << sx*sy << " and side: " << side << " and trees " << trees.size() << std::endl;
    return in*2 == trees.size();
}


int main() {
    while (cin.good() && !cin.eof()) {
    cin >> side;
    int64_t numTrees;
    cin >> numTrees;
    
    
    if (cin.eof())
	return 0;
	

	
    trees.resize(numTrees);
    for (size_t i = 0; i < trees.size(); ++i) {
	    int64_t x0, y0;
	    cin >> x0 >> y0;
	    trees[i].x = x0;
	    trees[i].y = y0;
    }

	//std::cout << numTrees << 'a' << std::endl;
    if ((numTrees % 2) == 1) {
	cout << -1 << endl;
	continue;
    }    
    bool found = false;
    int64_t minfound = 0;
    for (int64_t sx = 0; sx < side; ++sx) {
	for (int64_t sy = 0; sy < side; ++sy) {
	    if (found && sx*sy >= minfound)
		break;
	if (roh(0,0,sx, sy) || roh(0,1,sx, sy) || roh(1,0,sx, sy) || roh(1,1,sx,sy)) {
	    //cout << "FOUND: " << sx*sy << endl;
	    int64_t distfound = sx*sy;
	    if (!found || minfound > distfound) {
		minfound = distfound;
	    }
	    found = true;
	    continue;
	}
    }
    }
    if (found)
	cout << minfound << endl;
    else
	cout << -1 << endl;
    }
    return 0;
}