#include <fstream>

using namespace std;

ifstream f("hambar.in");
ofstream g("hambar.out");

pair <short, short> a[1002];
short n, m;

int smax;

short caut(short l1, short c1, short l2, short c2, short k) {
    while(k <= m)
        if(a[k].first >= l1 && a[k].first <= l2 && a[k].second <= c2 && a[k].second >= c1)
        	return k;
        else k++;
        return 0;
}
void div(short l1, short c1, short l2, short c2, short p) {
    int sa = (l2-l1+1)*(c2-c1+1);
    if(smax < sa) {
        p = caut(l1, c1, l2, c2, p);
        if(p == 0) { if(smax < sa) smax = sa; }
        else {
            div(l1, c1, l2, a[p].second - 1, p + 1);
            div(l1, a[p].second + 1, l2, c2, p + 1);
            div(l1, c1, a[p].first - 1, c2, p + 1);
            div(a[p].first + 1, c1, l2, c2, p + 1);
        }
    }
}

int main() {
    f >> n >> m;
    for(int i = 1; i <=m; ++i) f >> a[i].first >> a[i].second;
    div(1, 1, n, n, 1);
    g << smax;
    g.close();
    return 0;
}