#include <bits/stdc++.h>
#define MAXN 100005
#define INF 0x3f3f3f3f
using namespace std;
long long n, m, k, d[MAXN], viz[MAXN], s;
struct edge
{
    char special; // 0 - normal, 1 - special
    long long nod,c;
    edge(long long nod, long long c, char special):nod(nod),c(c),
    special(special) {}
    bool operator<(const edge &oth) const
    {
        if(c == oth.c) {
            if(special == 1)
                return -1;
        }
        return c>oth.c;
    }
};
vector<edge> v[MAXN];
priority_queue<edge> pq;
void djikstra(long long start)
{

    for(long long i=1; i<=n; ++i) {
        d[i]=INF;
        viz[i] = 0;
    }
    d[start]=0;
    pq.push(edge(start,0,0));
    while(!pq.empty())
    {
        long long nod = pq.top().nod;
        long long cost = pq.top().c;
        long long special = pq.top().special;
        //cout << "nod : " << nod << " special " << special << endl;
        pq.pop();
        if(viz[nod] == 0) {
            viz[nod] = 1;
            for(edge i : v[nod]) {
                if(d[nod] + i.c < d[i.nod]) {
                    d[i.nod] = d[nod] + i.c;
                    if(i.special) {
                        ++s;
                    }
                    pq.push(edge(i.nod, d[i.nod], 0));
                }
            }
        }
    }
}
int main()
{
    cin>>n>>m>>k;
    s = 0;
    for(long long i=1; i<=m; ++i)
    {
        long long a,b,c;
        cin>>a>>b>>c;
        v[a].push_back(edge(b, c, 0));
    }
    for(long long i = 1; i <= k; ++i)
    {
        long long a, b;
        cin >> a >> b;
        d[a] = b;
        v[1].push_back(edge(a, b, 1));
        // pq.push(edge(a,b,1));
    }
    djikstra(1);
    cout << k - s << endl;
    return 0;
}