#include <stdio.h>
#include <stdlib.h>
FILE *in,*out;
int d[50001],l[50001];
int main()
{
    in = fopen("orase.in","r");
    out = fopen("orase.out","w");
    int m,n,x,i,p=1,max = -2147483646;
    fscanf(in,"%d %d\n",&m,&n);
    for(i = 1;i <= n;i ++)
    {
        fscanf(in,"%d %d\n",&d[i],&l[i]);
    }
    for(i = 1;i <= n-1;i ++)
    {
        p = i + 1;
        while(p<=n)
        {
        if(d[p] > d[i])
        {
        if(l[i] + l[p] + d[p] - d[i] > max)
        {
            max = l[i] + l[p] + d[p] - d[i];
        }
        p ++;
        }
        else
        {
            if(l[i] + l[p] + d[i] - d[p] > max)
        {
            max = l[i] + l[p] + d[i] - d[p];
        }
        p ++;
        }

        }
    }
    fprintf(out,"%d",max);
    return 0;
}