#include <stdio.h>
#include <stdlib.h>

int main()
{
	
	long n,m,a[2][50001],s,s_max,i,j;
	
	freopen ("orase.in","r",stdin);
	freopen ("orase.out","w",stdout);
	
	scanf ("%ld %ld",&m,&n);
	
	for (i=1;i<=n;i++) scanf ("%ld %ld",&a[1][i],&a[2][i]);
	s_max=0;
	for (i=1;i<=n;i++) 
		for (j=i+1;j<=n;j++)
		{ if (a[1][j]>a[1][i])
			s=a[2][i]+a[2][j]+a[1][j]-a[1][i];
			    else s=a[2][i]+a[2][j]+a[1][i]-a[1][j];
		if (s>s_max) s_max=s;
		}
printf ("%ld",s_max);	
	

return 0;
}