#include <stdio.h>
#include <algorithm>

using namespace std;

#define maxim(a, b) ((a > b) ? a : b)
#define PII pair<int, int>

int M, N, bst;
PII v[50005];

int main(void)
{
    int i, j = -1000000000;
    
    freopen("orase.in", "r", stdin);
    freopen("orase.out", "w", stdout);

    scanf("%d %d", &M, &N);
    for (i = 1; i <= N; i++)
        scanf("%d %d", &v[i].first, &v[i].second);
        
    sort(v+1, v+N+1);
    for (i = 1; i <= N; i++)
    {
        bst = maxim(bst, v[i].first + v[i].second + j);
        j = maxim(j, v[i].second - v[i].first);
    }

    printf("%d\n", bst);
    
    return 0;
}