#include <fstream>

using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");
const int MMAX = 1000000;
int d[MMAX+5];
int main()
{
    int n, m, i, x, y, raspuns;
    fin>>m>>n;
    for(i =1 ; i <=n;i ++)
    {
        fin>>x>>y;
        d[x] = max(d[x], y);
    }
     raspuns = 0;
    for(i=1;i<=m;i++)
    {
       raspuns = max(raspuns, d[i-1] +1 + d[i]);
       d[i] = max(d[i-1] + 1, d[i]);
    }
    fout<<raspuns<<"\n";
    return 0;
}