#include <fstream>

using namespace std;

ifstream cin ("orase.in");
ofstream cout ("orase.out");

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

int main()
{
    int m,n;
    cin >> m >> n;
    for (int i=1; i<=n; i++)
    {
        cin >> v[i].first;
        cin >> v[i].second;
    }
    for (int i=1; i<=n-1; i++)
    {
        for (int j=i+1; j<=n; j++)
        {
            if (v[i].first>v[j].first)
            {
                pair <int, int> cp;
                cp=v[i];
                v[i]=v[j];
                v[j]=cp;
            }
        }
    }
    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;
        }
    }
    cout << sol;
    return 0;
}