#include<fstream>
using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");
int m,n,d,L,maxim,minim,maxim2,maxim3,i;
int main(){
    fin>>m>>n;
    minim=1000001;
    for(i=1;i<=n;i++){
        fin>>d>>L;
        if(maxim<=d && maxim2<L){
            maxim=d;
            maxim2=L;
        }
        if(minim>=d && maxim3<L){
            minim=d;
            maxim3=L;
        }
    }
    fout<<maxim-minim+maxim2+maxim3;
    return 0;
}