#include<cstdio>
#include<algorithm>
using namespace std;
struct structura
{
    int x;int y;
};
structura v[50001];
int i,indep,maxim,n,m;
bool cmp(structura a,structura b)
{
    return a.x<b.x;
}
int main()
{
    freopen("orase.in","r",stdin);
    freopen("orase.out","w",stdout);
    scanf("%d%d",&n,&m);
    for(i=1;i<=m;i++)
        scanf("%d%d",&v[i].x,&v[i].y);
    sort(v+1,v+m+1,cmp);
    scanf("%d%d",&v[1].x,&v[1].y);
    indep=1;
    for(i=2;i<=m;i++)
        {
            if(v[i].x-v[indep].x+v[i].y+v[indep].y>maxim)
                maxim=v[i].x-v[indep].x+v[i].y+v[indep].y;
            if(v[i].x-v[indep].x<v[i].y)
                indep=i;
        }
    printf("%d",maxim);
    return 0;
}