#include <iostream>
#include <cstdio>

using namespace std;

int main()
{
    FILE *fin, *fout;
    int m,n,i,st,dr,d,l;
    fin=fopen("orase.in","r");
    fout=fopen("orase.out","w");
    fscanf(fin,"%d %d",&m,&n);
    st=dr=0;
    for(i=1;i<=n;i++){
        fscanf(fin,"%d %d\n",&d,&l);
        st=max(st,d+l);
        dr=max(dr,l-d);
    }
    fprintf(fout,"%d\n",st+dr);
    fclose(fin);
    fclose(fout);
    return 0;
}