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

#define fi first
#define se second
#define pb push_back
#define FOR(i, a, b) for (int i = (a); i <= (b); ++i)
#define REP(i, n) for (int i = 0; i < (n); ++i)

typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, int> pli;

const int INF = 1e9;
const ll INF_LL = 1e18;

int n, m, num_s, max_t, st, en, K, nodes[105], id_st, id_en;
int d1[105][1005];
ll c[1005], f[105];
vector<pii> g1[1005];
vector<pli> g2[105];

int main() {
    ios_base::sync_with_stdio(0); cin.tie(0);
    if (!(cin >> n >> m >> num_s)) return 0;
    cin >> max_t;
    
    REP(i, m) {
        int u, v, w; cin >> u >> v >> w;
        g1[u].pb({v, w}); g1[v].pb({u, w});
    }
    
    FOR(i, 1, n) c[i] = INF_LL;
    REP(i, num_s) {
        int p; ll q; cin >> p >> q;
        c[p] = min(c[p], q);
    }
    cin >> st >> en;
    
    FOR(i, 1, n) if (c[i] != INF_LL || i == st || i == en) nodes[K++] = i;

    REP(i, K) {
        int start = nodes[i];
        FOR(j, 1, n) d1[i][j] = INF;
        priority_queue<pii, vector<pii>, greater<pii>> pq;
        d1[i][start] = 0; pq.push({0, start});
        
        while (!pq.empty()) {
            auto [u_d, u] = pq.top(); pq.pop();
            if (u_d > d1[i][u]) continue;
            for (auto& edge : g1[u]) {
                int v = edge.fi, w = edge.se;
                if (d1[i][u] + w < d1[i][v]) {
                    d1[i][v] = d1[i][u] + w;
                    pq.push({d1[i][v], v});
                }
            }
        }
    }
    
    REP(i, K) {
        int u = nodes[i]; if (c[u] == INF_LL) continue;
        REP(j, K) {
            int v = nodes[j];
            if (u != v && d1[i][v] <= max_t) g2[i].pb({j, 1LL * d1[i][v] * c[u]});
        }
    }
    
    id_st = -1, id_en = -1;
    REP(i, K) {
        if (nodes[i] == st) id_st = i;
        if (nodes[i] == en) id_en = i;
    }
    
    REP(i, K) f[i] = INF_LL;
    priority_queue<pli, vector<pli>, greater<pli>> pq;
    f[id_st] = 0; pq.push({0, id_st});
    
    while (!pq.empty()) {
        auto [u_d, u] = pq.top(); pq.pop();
        if (u_d > f[u]) continue;
        if (u == id_en) break;
        for (auto& edge : g2[u]) {
            int v = edge.fi; ll w = edge.se;
            if (f[u] + w < f[v]) {
                f[v] = f[u] + w; pq.push({f[v], v});
            }
        }
    }
    
    cout << f[id_en] << "\n";
    return 0;
}