#include <fstream>
#include <vector>
using namespace std;
ifstream fin ("orase.in");
ofstream fout ("orase.out");
int n,m,i,j,x,y,maxim,d;
int main()
{
    vector <int> v[1000001];

    fin >> n >> m;
    for(i=1;i<=m;i++)
    {
        fin >> x >>y;
        v[x].push_back(y);
    }

    for(i=0;i<=n;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++;
    }

        fout << maxim;
        return 0;
}