http://whu.contest.codeforces.com/group/LqmZzHyOQN/contest/234192/problem/F

题解：

莫名其妙的开瓶子？？？ 

/*
*@Author:   STZG
*@Language: C++
*/
#include <bits/stdc++.h>
#include<iostream>
#include<algorithm>
#include<cstdlib>
#include<cstring>
#include<cstdio>
#include<string>
#include<vector>
#include<bitset>
#include<queue>
#include<deque>
#include<stack>
#include<cmath>
#include<list>
#include<map>
#include<set>
//#define DEBUG
#define RI register int
using namespace std;
typedef long long ll;
//typedef __int128 lll;
const int N=10000;
const int MOD=1e9+7;
const double PI = acos(-1.0);
const double EXP = 1E-8;
const int INF = 0x3f3f3f3f;
int t,n,m,k,q;
int a[N];
int b[N];
int c[N];
int main()
{
#ifdef DEBUG
	freopen("input.in", "r", stdin);
	//freopen("output.out", "w", stdout);
#endif
    scanf("%d",&n);
    int ans=0;
    for(int i = 1; i <=n; i++){
        scanf("%d%d", &a[i],&b[i]);
    }
    for(int i = 1; i <=n; i++){
        for(int j = 1; j <=n; j++){
            if(b[i]==a[j]&&i!=j)
                c[j]=1;
        }
    }
    for(int i = 1; i <=n; i++){
        ans+=c[i];
    }
    cout << n-ans << endl;

    //cout << "Hello world!" << endl;
    return 0;
}