#include <fstream>
#include <algorithm>

#define MAX 50005

using namespace std;

struct punct
{
    int x, y;
}v[MAX];

int n, m;

bool cmp(punct a, punct b)
{
    return a.x < b.x;
}

int main()
{
    ifstream in("orase.in"); in>>m>>n;
    for(int i = 1; i <= n; i++)
        in>>v[i].x>>v[i].y;
    in.close(); sort(v + 1, v + n + 1, cmp);
    int dist = v[1].y, result = 0;
    for(int i = 2; i <= n; i++)
    {
        dist += (v[i].x - v[i - 1].x);
        result = max(result, dist + v[i].y);
        if(v[i].y > dist)
            dist = v[i].y;
    }
    ofstream out("orase.out"); out<<result; out.close();
    return 0;
}