#include <iostream>
#include <queue>

#define NMax 100010
#define pb push_back
#define INF 0x3f3f3f3f

using namespace std;

int n, m, k, u, v, sol, used[NMax];
bool relaxed[4 * NMax];
long long cost[4 * NMax], d[NMax], x;
priority_queue<pair<int, long long>> Q;
vector<pair<int, int>> G[NMax];

int main() {

    cin >> n >> m >> k;
    for (int i = 0; i < m; i++) {
        cin >> u >> v >> x;
        G[u].pb(make_pair(v, i));
        G[v].pb(make_pair(u, i));
        cost[i] = x;
    }

    for (int i = 0; i < k; i++) {
        cin >> u >> x;
        G[1].pb(make_pair(u, m + i));
        G[u].pb(make_pair(1, m + i));
        cost[m + i] = x;
    }

    for (int i = 1; i <= n; i++) {
        d[i] = INF;
        used[i] = -1;
    }
    d[1] = 0;
    Q.push(make_pair(1, 0));

    while (!Q.empty()) {
        int nod = Q.top().first;
        long long dist = -Q.top().second;
        Q.pop();

        if(dist > d[nod]) {
            continue;
        }

        for (auto x : G[nod]) {
            int adj = x.first;
            int edge = x.second;

            if (d[adj] > dist + cost[edge]) {
                d[adj] = dist + cost[edge];
                used[adj] = edge;
                Q.push(make_pair(adj, -d[adj]));
            } else if (d[adj] == dist + cost[edge] && edge < m) {
                used[adj] = edge;
            }
        }
    }
    for (int i = 1; i <= n; i++) {
        if (used[i] != -1) {
            relaxed[used[i]] = true;
        }
    }
    for (int i = m; i < m + k; i++) {
        if (!relaxed[i]) {
            sol++;
        }
    }
    cout << sol << '\n';
    return 0;
}
close