#include <fstream>
#include <algorithm>

using namespace std;

ifstream in("orase.in");
ofstream out("orase.out");
int m, n, ans;
pair<int, int> v[50005];

int main()
{
    in>>m>>n;
    for(int i = 1; i<=n; i++)
    {
        in>>v[i].first>>v[i].second;
    }

    sort(v + 1, v + n + 1);

    int best = 0;
    for(int i = 1; i<=n; i++)
    {
        if(i != 1)
        {
            best += v[i].first - v[i - 1].first;
        }

        ans = max(ans, best + v[i].second);

        best = max(best, v[i].second);
    }

    out<<ans;

    return 0;
}