use proconio::{input, marker::Usize1};
use std::collections::VecDeque;

fn main() {
    input! {
        n: usize,
        m: usize,
        p: usize,
        y: usize,
        abc: [(Usize1, Usize1, usize); m],
        de: [(Usize1, usize); p],
    }

    let mut graph = vec![vec![]; n];
    for (a, b, c) in abc {
        graph[a].push((b, c));
        graph[b].push((a, c))
    }

    let mut rest_money = vec![0; n];
    let mut queue = VecDeque::new();
    queue.push_back((0, y));
    while let Some((pos, money)) = queue.pop_front() {
        for &(next, robbed) in &graph[pos] {
            if rest_money[next] >= money - robbed {
                continue;
            }

            rest_money[next] = money - robbed;
            queue.push_back((next, money - robbed));
        }
    }

    let mut ans = 0;
    for (d, e) in de {
        ans = ans.max(rest_money[d] / e);
    }
    println!("{}", ans);
}