#include <fstream>
#include <vector>

using namespace std;
ifstream f("orase.in");
ofstream g("orase.out");

int maxim,d,D,L,i,j,M,n;

vector <int> v[1000003];

int main()
{
    f >> M >> n;
    for(i=0;i<n;i++){f >> D >> L; v[D].push_back(L);}

    for(i=0;i<=M;i++){
        for(j=0;j<v[i].size();j++){
            if(v[i][j]!=0&&i!=0) maxim = max(maxim,d + v[i][j]);
        if(d<v[i][j]) d = v[i][j];}
        d++;}
    g << maxim;
    return 0;
}