#include<stdio.h>
#include<stdlib.h>
FILE*f=fopen("orase.in","r");
FILE*g=fopen("orase.out","w");
long n,m,d[50009],l[50009],dmax=-100;
void read()
        {
        int i,j;
        fscanf(f,"%ld %ld",&m,&n);
        for(i=1;i<=n;++i)
        fscanf(f,"%ld %ld",&d[i],&l[i]);
        }
void solve()
        {
        int i,j;
        for(i=1;i<n;++i)
            for(j=i+1;j<=n;++j)
                  if(abs(d[i]-d[j])+l[i]+l[j]>dmax) dmax=abs(d[i]-d[j])+l[i]+l[j];
       fprintf(g,"%ld",dmax);
       }
int main()
        {
        read();
        solve();
        fclose(f);
        fclose(g);
        return 0;
        }