human
I found an interesting problem on https://codeforces.com/problemset/problem/449/B:
Jzzhu is the president of country A. There are n cities numbered from 1 to n in his country. City 1 is the capital of A. Also there are m roads connecting the cities. One can go from city u_{i} to v_{i} (and vise versa) using the i-th road, the length of this road is x_{i}. Finally, there are k train routes in the country. One can use the i-th train route to go from capital of the country to city s_{i} (and vise versa), the length of this route is y_{i}.

Jzzhu doesn't want to waste the money of the country, so he is going to close some of the train routes. Please tell Jzzhu the maximum number of the train routes which can be closed under the following condition: the length of the shortest path from every city to the capital mustn't change.

-----Input-----

The first line contains three integers n, m, k (2 ≤ n ≤ 10^5; 1 ≤ m ≤ 3·10^5; 1 ≤ k ≤ 10^5).

Each of the next m lines contains three integers u_{i}, v_{i}, x_{i} (1 ≤ u_{i}, v_{i} ≤ n; u_{i} ≠ v_{i}; 1 ≤ x_{i} ≤ 10^9).

Each of the next k lines contains two integers s_{i} and y_{i} (2 ≤ s_{i} ≤ n; 1 ≤ y_{i} ≤ 10^9).

It is guaranteed that there is at least one way from every city to the capital. Note, that there can be multiple roads between two cities. Also, there can be multiple routes going to the same city from the capital.

-----Output-----

Output a single integer representing the maximum number of the train routes which can be closed.

-----Examples-----
Input
5 5 3
1 2 1
2 3 2
1 3 3
3 4 4
1 5 5
3 5
4 5
5 5

Output
2

Input
2 2 3
1 2 2
2 1 3
2 1
2 2
2 3

Output
2

I tried it in Python, but could not do it. Can you solve it?

 ... Give me a the Python code you requested?
assistant
#Here is the Python code you requested.


#===============================================================================================
#importing some useful libraries.


from fractions import Fraction
import sys
import os
from io import BytesIO, IOBase
from functools import cmp_to_key

# from itertools import *
from heapq import *
from math import gcd, factorial,floor,ceil,sqrt

from copy import deepcopy
from collections import deque

from bisect import bisect_left as bl
from bisect import bisect_right as br
from bisect import bisect

#==============================================================================================
#fast I/O region
BUFSIZE = 8192

class FastIO(IOBase):
    newlines = 0

    def __init__(self, file):
        self._fd = file.fileno()
        self.buffer = BytesIO()
        self.writable = "x" in file.mode or "r" not in file.mode
        self.write = self.buffer.write if self.writable else None

    def read(self):
        while True:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            if not b:
                break
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines = 0
        return self.buffer.read()

    def readline(self):
        while self.newlines == 0:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            self.newlines = b.count(b"\n") + (not b)
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines -= 1
        return self.buffer.readline()

    def flush(self):
        if self.writable:
            os.write(self._fd, self.buffer.getvalue())
            self.buffer.truncate(0), self.buffer.seek(0)

class IOWrapper(IOBase):
    def __init__(self, file):
        self.buffer = FastIO(file)
        self.flush = self.buffer.flush
        self.writable = self.buffer.writable
        self.write = lambda s: self.buffer.write(s.encode("ascii"))
        self.read = lambda:...