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

template<class T>
bool chmax(T& a, const T& b) {
    if (a < b) { a = b; return true; }
    return false;
}
template<class T>
bool chmin(T& a, const T& b) {
    if (b < a) { a = b; return true; }
    return false;
}

// std::vector Declaration
template<typename T>
vector<T> make_v(size_t a) { return vector<T>(a); }
template<typename T, typename... Ts>
auto make_v(size_t a, Ts... ts) {
    return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}

// std::vector Declaration and Initialization
template<typename T>
vector<T> make_vector(size_t a, T x) { return vector<T>(a, x); }
template<typename T, typename U, typename... Ts>
auto make_vector(size_t a, U b, Ts... ts) {
    return vector<decltype(make_vector<T>(b,ts...))>(a, make_vector<T>(b, ts...));
}

// std::vector Input
template<typename T>
istream& operator>>(istream& is, vector<T>& v) {
    for (auto &e : v) is >> e;
    return is;
}

// std::vector Debug
template<typename T>
ostream& operator<<(ostream& os, const vector<T>& v) {
    os << "[";
    bool a = 1;
    for (auto e : v) {
        os << (a ? "" : " ");
        os << e;
        a = 0;
    }
    os << "]";
    return os;
}

// std::array Debug
template<typename T, size_t n>
ostream& operator<<(ostream& os, const array<T, n>& v) {
    os << "[";
    bool a = 1;
    for (auto e : v) {
        os << (a ? "" : " ");
        os << e;
        a = 0;
    }
    os << "]";
    return os;
}

// std::deque Debug
template<typename T>
ostream& operator<<(ostream& os, const deque<T>& d) {
    os << "[";
    bool a = 1;
    for (auto e : d) {
        os << (a ? "" : " ");
        os << e;
        a = 0;
    }
    os << "]";
    return os;
}

// std::pair Debug
template<typename T, typename U>
ostream& operator<<(ostream& os, const pair<T, U>& p) {
    os << "(" << p.first << " " << p.second << ")";
    return os;
}

// std::set Debug
template<typename T>
ostream& operator<<(ostream& os, const set<T>& st) {
    os << "{";
    bool a = 1;
    for (auto e : st) {
        os << (a ? "" : " ");
        os << e;
        a = 0;
    }
    os << "}";
    return os;
}

// std::multiset Debug
template<typename T>
ostream& operator<<(ostream& os, const multiset<T>& st) {
    os << "{";
    bool a = 1;
    for (auto e : st) {
        os << (a ? "" : " ");
        os << e;
        a = 0;
    }
    os << "}";
    return os;
}

// std::map Debug
template<typename T, typename U>
ostream& operator<<(ostream& os, const map<T, U>& mp) {
    os << "{";
    bool a = 1;
    for (auto e : mp) {
        os << (a ? "" : " ");
        os << e.first << ":" << e.second;
        a = 0;
    }
    os << "}";
    return os;
}

// std::tuple Debug
template<int N, class Tuple>
void out(ostream& os, const Tuple& t){}
template<int N, class Tuple, class H, class ...Ts>
void out(ostream& os, const Tuple& t) {
    if (N) os << " ";
    os << get<N>(t);
    out<N+1,Tuple,Ts...>(os, t);
}
template<class ...Ts>
ostream& operator<<(ostream& os, const tuple<Ts...>& t) {
    os << "(";
    out<0,tuple<Ts...>,Ts...>(os, t);
    os << ")";
    return os;
}

// Debug
#define DUMP(x) cerr<<#x<<" = "<<(x)<<endl

// Weighted edge
template<typename T>
struct edge {
    //int src, to;
    int to;
    T cost;

    edge() {}
    //edge(int to, T cost) : src(-1), to(to), cost(cost) {}
    edge(int to, T cost) : to(to), cost(cost) {}
    //edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}

    friend ostream& operator<<(ostream& os, const edge& e) {
        return os << "(" << e.src << "->" << e.to << ":" << e.cost << ")";
    }
};

using LL = int64_t;

#define fs first
#define sc second

const int64_t MOD = 1e9+7;


template<typename T>
vector<vector<T>> warshall_floyd(vector<vector<T>> g) {
    const auto INF = numeric_limits<T>::max();
    int n = g.size();
    for (int k = 0; k < n; k++) {
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                if (g[i][k] == INF or g[k][j] == INF) continue;
                g[i][j] = min(g[i][j], g[i][k] + g[k][j]);
            }
        }
    }
    return g;
}

int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);

    while (true) {
        int n, m, l, s, t;
        cin >> n >> m >> l >> s >> t; --s;
        if (n == 0) break;
        vector<vector<edge<int>>> G(n);
        for (int i = 0; i < m; ++i) {
            int a, b, c; cin >> a >> b >> c; --a, --b;
            G[a].emplace_back(b, c);
            G[b].emplace_back(a, c);
        }
        vector<int> j(l+1), e(l+1);
        for (int i = 0; i < l; ++i) {
            cin >> j[i] >> e[i]; --j[i];
        }
        j[l] = s, e[l] = 0;
        ++l;

        const LL INF = numeric_limits<LL>::max();
        auto mat = make_vector<LL>(n, n, INF);
        for (int i = 0; i < n; ++i) {
            mat[i][i] = 0;
            for (const auto& eg : G[i]) {
                mat[i][eg.to] = eg.cost;
            }
        }
        mat = warshall_floyd(mat);
        //for (int i = 0; i < n; ++i) {
        //    DUMP(mat[i]);
        //}

        auto dp = make_vector<LL>(l, 1<<l, INF);
        dp[l-1][0] = 0;
        //for (int i = 0; i < l; ++i) {
        //    DUMP(dp);
        //}
        for (int bit = 0; bit < 1<<l; ++bit) {
            for (int u = 0; u < l; ++u) {
                if (dp[u][bit] == INF) {
                    continue;
                }
                for (int v = 0; v < l; ++v) if (u != v) {
                    if (mat[j[u]][j[v]] == INF) {
                        continue;
                    }
                    chmin(dp[v][bit | 1<<v],
                            dp[u][bit] + mat[j[u]][j[v]] + e[v]);
                }
            }
        }
        //for (int i = 0; i < l; ++i) {
        //    DUMP(dp);
        //}

        int ans = 0;
        for (int bit = 0; bit < 1<<l; ++bit) {
            if (dp[l-1][bit] <= t) {
                chmax(ans, __builtin_popcount(bit) - 1);
            }
        }
        cout << ans << endl;
    }

    return 0;
}



Measured memory_bytes: 22944