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

using namespace std;

#define maxn 50010
#define maxv 10000000

int n,m,sol;
int a[maxn],b[maxn],p[maxn];

int cmp(int x,int y)
{
    return a[x]<a[y];
}

int main()
{
    freopen("orase.in","r",stdin);
    freopen("orase.out","w",stdout);
    
    scanf("%d %d ",&m,&n);
    
    int i,min=maxv;
    
    for (i=1;i<=n;i++) 
    {
        scanf("%d %d ",&a[i],&b[i]);
        p[i]=i;
    }
    
    sort(p+1,p+n+1,cmp);
    
    for (i=1;i<=n;i++)
    {
        if (a[p[i]]+b[p[i]]-min>sol) sol=a[p[i]]+b[p[i]]-min;
        if (a[p[i]]-b[p[i]]<min) min=a[p[i]]-b[p[i]];
    }
    
    printf("%d\n",sol);
    
    return 0;
}