#include <bits/stdc++.h>
#define ll long long
#define db long double
#define inf LLONG_MAX / 2

using namespace std;

vector<ll> dijk(int N, vector<vector<pair<int, int>>> &adj, int s) {
    vector<ll> cost(N, inf);
    priority_queue<pair<ll, int>> q;
    q.push({0, s});
    while (!q.empty()) {
        auto t = q.top();
        q.pop();
        if (cost[t.second] != inf) continue;
        cost[t.second] = -t.first;
        for (pair<int, int> e: adj[t.second]) {
            q.push({t.first - e.second, e.first});
        }
    }
    return cost;
}

int main() {
    int N, M, K;
    cin >> N >> M >> K;
    vector<vector<pair<int, int>>> adj(N);
    for (int i = 0; i < M; i++)
    {
        int u, v, w;
        cin >> u >> v >> w;
        u--; v--;
        adj[u].push_back({v, w});
        adj[v].push_back({u, w});
    }

    vector<db> p(N);
    while (K--) {
        int i;
        db d;
        cin >> i >> d;
        p[i - 1] = d;
    }
    
    vector<ll> ch = dijk(N, adj, 0);
    vector<ll> cu = dijk(N, adj, N - 1);
    vector<ll> c(N);
    for (int i = 0; i < N; i++)
    {
        c[i] = min(inf, ch[i] + cu[i]);
    }
    
    for (int i = 0; i < N; i++)
    {
        if (p[i] == 1 && c[i] != inf) goto yes;
    }

    cout << "impossible" << endl;
    return 0;
    
    yes:
    vector<pair<ll, db>> paths;
    for (int i = 0; i < N; i++)
    {
        paths.push_back({c[i], p[i]});
    }
    sort(paths.begin(), paths.end());

    db r = 1;
    db e = 0;
    for (auto i: paths) {
        e += r * i.second * i.first;
        r *= 1 - i.second;
    }

    cout << e << endl;
}