#include <bits/stdc++.h>
#define MAX 100005
#define INF 1e9 * 100005
using namespace std;

int n, m, k;
vector <pair<int, long long> > adj[MAX];
bool reached[MAX];

int getSP() {
    vector<long long> dist(MAX, INF);
    priority_queue<pair<long long, int>, vector<pair<long long, int> >, greater<pair<long long, int> > > pq;
    pq.push(make_pair(0, 0));
    dist[0] = 0;
    for (int i = 0; i < k; i++) {
        int s; long long y;
        scanf("%d %lld", &s, &y);
        s--;
        dist[s] = min(dist[s], y);
        if (!reached[s]) {
            reached[s] = true;
            pq.push(make_pair(dist[s], s));
        }
    }
    while (!pq.empty()) {
        int u = pq.top().second;
        pq.pop();
        for (int i = 0; i < adj[u].size(); i++) {
            int v = adj[u][i].first;
            long long w = adj[u][i].second;
            if (dist[v] == INF) {
                dist[v] = dist[u] + w;
                pq.push(make_pair(dist[v], v));
            }
            else if (dist[v] >= dist[u] + w) {
                dist[v] = dist[u] + w;
                reached[v] = false;
                pq.push(make_pair(dist[v], v));
            }
        }
    }
    int ans = k;
    for (int i = 0; i < n; i++)
        if (reached[i])
            ans--;
    return ans;
}

int main() {
    scanf("%d %d %d", &n, &m, &k);
    for (int i = 0; i < m; i++) {
        int u, v;
        long long x;
        scanf("%d %d %lld", &u, &v, &x);
        u--, v--;
        adj[u].push_back(make_pair(v, x));
        adj[v].push_back(make_pair(u, x));
    }
    cout << getSP();
}