#include <stdio.h>

#include <algorithm>



using namespace std;



#define MAX_N 50005

#define FIN "orase.in"

#define FOUT "orase.out"

#define f first

#define s second



int M, N, Res;

pair<int, int> A[MAX_N];



int main(void)

{

    int i, bst;



    freopen(FIN, "r", stdin);

    freopen(FOUT, "w", stdout);



    scanf("%d %d", &M, &N);

    for (i = 0; i < N; i++)

        scanf("%d %d", &A[i].f, &A[i].s);



    sort(A, A+N);

    bst = A[0].f-A[0].s;

    for (i = 1; i < N; i++)

    {

        Res = max(Res, A[i].f+A[i].s-bst);

        bst = min(bst, A[i].f-A[i].s);

    }

    printf("%d\n", Res);



    return 0;

}