#include <fstream>
#include <vector>

using namespace std;

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

int n, m, maxim, d, di, li, i, j;

vector <int> v[1000000];

int main()
{
    f >> n >> m;

    for( i = 1 ; i <= m; i ++)
    {
        f >> di >> li;
        v[di].push_back(li);
    }

    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++;
    }

    g << maxim;

    return 0;
}