#include <fstream>

using namespace std;

int main(){
    ifstream fin("orase.in");
    ofstream fout("orase.out");
    int m,n,nr1,nr2,max1=-1,max2=-1,i;
    fin>>m>>n;
    for(i=1;i<=n;i++){
        fin>>nr1>>nr2;
        if(nr1+nr2>max1){
            max1=nr1+nr2;
        }
        if(nr2-nr1>max2){
            max2=nr2-nr1;
        }
    }
    fout<<max1+max2;
    return 0;
}