#include <fstream>
#include <algorithm>

using namespace std;

ifstream ci ("orase.in");
ofstream co ("orase.out");

const int nmax=50000;
pair <int ,int> v[nmax+1];

int main()
{
    int m,n;
    ci >> m >> n;
    for (int i=1; i<=n; i++)
    {
        ci >> v[i].first;
        ci >> v[i].second;
    }
    sort(v+1,v+n+1);
    int max=v[1].second,sol=0;
    for (int i=2; i<=n; i++)
    {
        max+=v[i].first-v[i-1].first;
        if (v[i].second+max>sol)
        {
            sol=v[i].second+max;
        }
        if (v[i].second>max)
        {
            max=v[i].second;
        }
    }
    co << sol;
    return 0;
}