#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;
}