#include <fstream>

using namespace std;

ifstream fin("orase.in");
ofstream fout("orase.out");

const int nmax=50000;

int d[nmax+1], l[nmax+1];

int main(){
    int m,n;
    fin>>m>>n;
    for(int i=1;i<=n;i++){
        fin>>d[i]>>l[i];
    }
    int sol=0,p=1;
    for(int i=1;i<=n;i++){
        if(l[i]-d[i]>l[p]-d[p]){
            p=i;
        }
    }
    for(int i=1;i<=n;i++){
        int sum=l[i]+d[i]-d[p]+l[p];
        if(sum>sol){
            sol=sum;
        }
    }
    fout<<sol<<"\n";
    return 0;
}