#include <stdio.h>
#include <math.h>
#define nmax 50*1000+1

long int M, N, D[nmax], L[nmax], MAX;

int main()
{
    long int i,j;
    long int rez;
    FILE *in, *ie;
    in=fopen("orase.in","r");
    fscanf(in,"%ld %ld",&M,&N);
    for(i=0;i<N;i++)
    {
        fscanf(in,"%ld %ld\n",&D[i],&L[i]);
    }
    fclose(in);
    for(i=0;i<N;i++)
    for(j=0;j<N;j++)
    {
        if((L[i]+L[j]+fabs(D[i]-D[j]))>MAX)
        MAX=rez;
    }
    ie=fopen("orase.out","w");
    fprintf(ie,"%ld",MAX);
    fclose(ie);
}