package main

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

func main() {
	in := bufio.NewReader(os.Stdin)

	var n int
	fmt.Fscan(in, &n)

	a := make([]int, n)
	canOpen := make(map[int]bool)

	for i := 0; i < n; i++ {
		var ai, bi int
		fmt.Fscan(in, &ai, &bi)
		a[i] = ai
		canOpen[bi] = true
	}

	ans := 0
	for i := 0; i < n; i++ {
		if !canOpen[a[i]] {
			ans++
		}
	}

	fmt.Println(ans)
}