#include<bits/stdc++.h>
using namespace std;

int n,m,al,ar = INT_MAX, bl,br = INT_MAX;

int main(){
	ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0);
	
	cin >> n;
	for(int i = 1; i <= n; i++){
		int l,r;
		cin >> l >> r;
		al = max(al,l);
		ar = min(ar,r);
	}
	cin >> m;
	for(int i = 1; i <= m; i++){
		int l,r;
		cin >> l >> r;
		bl = max(bl,l);
		br = min(br,r);
	}
	
	cout << max({al-br,bl-ar,0});
	
	return 0;
}