#include <fstream>
#include <vector>
using namespace std;
ifstream f("orase.in");
ofstream g("orase.out");

vector <int>v[1000001];
int m,n,i,j,x,y,maxim,d;
int main()
{
    f>>m>>n;
    for(i=1;i<=n;i++)
    {
        f>>x>>y;
        v[x].push_back(y);
    }
    for(i=0;i<=m;i++)
    {
        for(j=0;j<v[i].size();j++)
        {
            if(v[i][j]!=0)
               maxim=max(maxim,d+v[i][j]);
            if(d<v[i][j])
                d=v[i][j];
        }
        d++;
    }
    g<<maxim;
    return 0;
}