#1972
#include <fstream>
using namespace std;
ifstream fin("hambar.in");
ofstream fout("hambar.out");
int a[1002][2];
int n,m,k=1,i;
int smax=0;
int cauta(int l1, int c1, int l2, int c2, int k)
{
    while(k<=m)
        if(a[k][0]>=l1 and a[k][0]<=l2 and a[k][1]>=c1 and a[k][1]<=c2)
            return k;
        else
            k++;
    return 0;
}


void div(int l1, int c1, int l2, int c2, int p)
{
    int sa=(l2-l1+1)*(c2-c1+1);
    if(smax<sa)
    {
        p=cauta( l1, c1, l2, c2, p);
        if(p==0)
            smax=max(smax, sa);
        else
        {
            div(l1, c1, l2, a[p][1]-1,p+1);
            div(l1, a[p][1]+1, l2, c2,p+1);
            div(l1, c1, a[p][0]-1, c2,p+1);
            div(a[p][0]+1, c1, l2, c2,p+1);
        }
    }
}
int main()
{

    fin>>n>>m;
    for(i=1; i<=m; i++)
        fin>>a[i][0]>>a[i][1];
    div(1,1,n,n,1);
    fout<<smax;
    return 0;
}