/*
Punctaj final pe sursa curenta: 100p
*/
#include <iostream>
#include <fstream>
#include <algorithm>

using namespace std;
 
ifstream f("orase.in");
ofstream g("orase.out");
 
struct oras {
    int d, l;
};
 
inline bool cmp(oras a, oras b) {
    return a.d < b.d;
}
 
int n, m;
oras orase[50005];
int dmax;
int s, mx;
 
int main()
{
    f >> m >> n;
 
    for (int i = 1; i <= n; i++) {
        f >> orase[i].d >> orase[i].l;
    }
 
    sort(orase + 1, orase + n + 1, cmp);
 
    for (int i = 1; i <= n; i++)
    {
        if (s < mx + orase[i].d + orase[i].l)
            s = mx + orase[i].d + orase[i].l;
        if (mx < orase[i].l - orase[i].d)
            mx = orase[i].l - orase[i].d;
    }
 
    g << s;
 
    return 0;
}