#include <cstdio>
#include <cmath>
#include <algorithm>
using namespace std;

FILE *f=fopen("orase.in", "r");
FILE *g=fopen("orase.out", "w");

struct date{
    int d, l;
} a[50001];

int n, m, i, maxx=-2147483647, w=-2147483647;

bool cmp(date a, date b){
    return(a.d<b.d);
}

int main()
{
    fscanf(f, "%d%d", &m, &n);
    for(int i=1; i<=n; ++i)
        fscanf(f, "%d%d", &a[i].d, &a[i].l);
    sort(a+1, a+n+1, cmp);
    for(int i=1; i<=n; ++i){
        maxx=max(maxx, a[i].d+a[i].l+w);
        w=max(w, a[i].l-a[i].d);
    }
    fprintf(g, "%d", maxx);
return 0;
}