#include <stdio.h>
#include <stdlib.h>

#define IN "orase.in"
#define OUT "orase.out"
#define NMAX 50001

struct CITY{
    int dist, len;
} v[NMAX];

int n, m;

void read(void){
    int i;
    scanf ("%d%d", &m, &n);
    for (i = 1; i <= n; ++ i)
        scanf ("%d%d", &v[i].dist, &v[i].len);
}

inline void swap (struct CITY *a, struct CITY *b){
    struct CITY aux;
    aux = *a;
    *a = *b;
    *b = aux;
}

void quickSort (int left, int right){
    int i, j, pivot;
    i = left, j = right, pivot = v[(left + right) / 2].dist;

    while (i <= j){
        while (v[i].dist < pivot)
            ++ i;
        while (v[j].dist > pivot)
            -- j;

        if (i <= j){
            swap (&v[i], &v[j]);
            ++ i;
            -- j;
        }
    }
    if (left <= j)
        quickSort (left, j);
    if (i <= right)
        quickSort (i, right);
}

inline int max (int a, int b){
    if (a >= b)
        return a;
    return b;
}

int best (void){
    int sum, bestSum, i, sum1, sum2;
    sum = 0, bestSum = 0;

    for (i = 2; i <= n; ++i){
        sum1 = v[i - 1].len - v[i - 1].dist + v[i].dist + v[i].len;
        sum2 = sum + v[i].dist + v[i].len - v[i - 1].len - v[i - 1].dist;
        sum = max (sum1, sum2);

        if (bestSum < sum)
            bestSum = sum;
    }

    return bestSum;
}

int main(void){
    freopen (IN, "r", stdin);
    freopen (OUT, "w", stdout);

    read ();
    quickSort (1, n);
    printf ("%d", best());

    fclose (stdin);
    fclose (stdout);
    return 0;
}