codeforces Round #274(div2) C解題報告

C. Exams

time limit per test

1 second

memory limit per test

256 megabytes

input

standard input

output

standard output

Student Valera is an undergraduate student at the University. His end of term exams are approaching and he is to pass exactly n exams. Valera is a smart guy, so he will be able to pass any exam he takes on his first try. Besides, he can take several exams on one day, and in any order.

According to the schedule, a student can take the exam for the i-th subject on the day number ai. However, Valera has made an arrangement with each teacher and the teacher of the i-th subject allowed him to take an exam before the schedule time on day bi (bi < ai). Thus, Valera can take an exam for the i-th subject either on day ai, or on day bi. All the teachers put the record of the exam in the student's record book on the day of the actual exam and write down the date of the mark as number ai.

Valera believes that it would be rather strange if the entries in the record book did not go in the order of non-decreasing date. Therefore Valera asks you to help him. Find the minimum possible value of the day when Valera can take the final exam if he takes exams so that all the records in his record book go in the order of non-decreasing date.

Input

The first line contains a single positive integer n (1 ≤ n ≤ 5000) — the number of exams Valera will take.

Each of the next n lines contains two positive space-separated integers ai and bi (1 ≤ bi < ai ≤ 109) — the date of the exam in the schedule and the early date of passing the i-th exam, correspondingly.

Output

Print a single integer — the minimum possible number of the day when Valera can take the last exam if he takes all the exams so that all the records in his record book go in the order of non-decreasing date.

Sample test(s)

input

3
5 2
3 1
4 2

output

2

input

3
6 1
5 2
4 3

output

6

Note

In the first sample Valera first takes an exam in the second subject on the first day (the teacher writes down the schedule date that is 3). On the next day he takes an exam in the third subject (the teacher writes down the schedule date, 4), then he takes an exam in the first subject (the teacher writes down the mark with date 5). Thus, Valera takes the last exam on the second day and the dates will go in the non-decreasing order: 3, 4, 5.

In the second sample Valera first takes an exam in the third subject on the fourth day. Then he takes an exam in the second subject on the fifth day. After that on the sixth day Valera takes an exam in the first subject.

題目大意：

      Valera參加期末考試，某場考試i，學校的安排是第ai天考，但老師可以讓他bi天考，但老師在表格上填寫的還是第ai天考。Valera想要表格上填寫的是單調遞增序列，並且最早考完，Valera很聰明，每門課都可以過，並且一天可以考多門。

解法：

由於寫到表格上的日期始終是ai，要想表格上的日期單調不遞減，就必須要按照ai單調不遞減的序列的順序來考試。

同時真正考試的日期，也必須要是單調不遞增序列。

這道題目就化成在ai單調不遞減的情況下，構造出一個每個i-th由{ai, bi}其中一個組成的單調不遞減序列。

代碼：

#include <cstdio>
#include <algorithm>
#define N_max 5010

using namespace std;

struct typ1{
	int a, b;
}d[N_max];

int ans, n;

bool cmp(typ1 a, typ1 b) {
	if (a.a < b.a)  
		return true;
	else if (a.a == b.a && a.b < b.b)
		return true;

	return false;
}

void init() {
	scanf("%d", &n);

	for (int i = 1; i <= n; i++)
		scanf("%d%d", &d[i].a, &d[i].b);

	sort(d+1, d+n+1, cmp);
}

void solve() {
	ans = 0;

	for (int i = 1; i <= n; i++)
		if (d[i].b >= ans)
			ans = d[i].b;
		else
			ans = d[i].a;

	printf("%d\n", ans);
}

int main() {
	init();
	solve();
}