///   InfoArena ~~ Orase

#include<fstream>
#include<algorithm>

using namespace std;

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

const int NMAX = 50002;

struct oras { int d, l; };
oras O[NMAX];

int N,M, sol;

bool comp( const oras &unu, const oras &doi ) {
    return unu.d < doi.d;
}

int main() {
    int ormin;
    in >> M >> N;

    for( int i = 1;  i <= N;  i++ ) {
        in >> O[i].d >> O[i].l;
    }

    sort(O+1, O+N+1, comp);

    ormin = 1;
    for( int i = 2;  i <= N;  i++ ) {
        sol = max(sol, O[i].d + O[i].l + O[ormin].l - O[ormin].d);
        if((O[i].l - O[i].d) > (O[ormin].l - O[ormin].d)) {
            ormin = i;
        }
    }

    out << sol;

    return 0;
}