package main

import (
	"bufio"
	"fmt"
	"os"
)

func main() {
	in := bufio.NewReader(os.Stdin)
	out := bufio.NewWriter(os.Stdout)
	defer out.Flush()

	var n int
	if _, err := fmt.Fscan(in, &n); err != nil {
		return
	}

	a := make([]int, n)
	b := make([]int, n)
	for i := 0; i < n; i++ {
		fmt.Fscan(in, &a[i], &b[i])
	}

	ans := 0
	for i := 0; i < n; i++ {
		openable := false
		for j := 0; j < n; j++ {
			if i != j && b[j] == a[i] {
				openable = true
				break
			}
		}
		if !openable {
			ans++
		}
	}

	fmt.Fprintln(out, ans)
}