#include <stdio.h>
#include <stdlib.h>
int v[1000001];
int main()
{
    int m,n,i,di,li,max,nr;
    FILE *fin,*fout;
    fin=fopen("orase.in","r");
    fout=fopen("orase.out","w");
    fscanf(fin,"%d %d",&m,&n);
    for(i=0;i<n;i++){
        fscanf(fin,"%d %d",&di,&li);
        if(v[di]<li)
            v[di]=li;
    }
    nr=0;
    max=0;
    for(i=1;i<=m;i++){
        if((i-nr)+v[nr]+v[i]>max)
            max=(i-nr)+v[nr]+v[i];
        if(v[i]+nr>v[nr]+i)
            nr=i;
    }
    fprintf(fout,"%d",max);
    fclose(fin);
    fclose(fout);
    return 0;
}