#include <iostream>
#include <fstream>
using namespace std;
ifstream f("hambar.in");
ofstream g("hambar.out");
int main()
{
int gradina[100][100]={0,0}, N, M, S=0, x, y, i, j, l, c, ok, nule;
f>>N>>M;
for(i=1;i<=M;i++){
f>>x>>y; gradina[x][y]=1;
}
for(i=1;i<N;i++)
for(j=1;j<N;j++)
if(gradina[i][j]==0)
{
l=i; c=j;
while(gradina[i][c+1]==0 && c+1<=N)
c++;
while(gradina[l+1][j]==0 && l+1<=N)
l++;
nule=0;
for(x=i;x<=l;x++)
for(y=j;y<=c;y++)
if(gradina[x][y]==0)
nule++;
if(c-j+1>1 && l-i+1>1 && (c-j+1)*(l-i+1)==nule)
if(S<nule)
S=nule;
j=c;
}
g<<S;
return 0;
}