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

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];
    rest_money[0] = y;
    let mut heap = BinaryHeap::new();
    heap.push((y, 0));

    while let Some((money, pos)) = heap.pop() {
        if rest_money[pos] > money {
            continue;
        }

        for &(next, robbed) in &graph[pos] {
            let new_money = (money - robbed).max(0);
            if new_money == 0 || rest_money[next] >= new_money {
                continue;
            }

            rest_money[next] = new_money;
            heap.push((rest_money[next], next));
        }
    }

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