1414 - Find the absent number

Time Limit: 10s Memory Limit: 128MB

Submissions: 235 Solved: 92
Description

Yesterday xiaoY’s teacher HH had given him an interesting problem, and xiaoY got mad about this problem. So he turned to you for help, here is the problem!

HH gave xiaoY a number M and N intervals [xi,yi] which means the integers from xi to yi are all chosen, Now he wanted xiaoY to tell him which number was absent!

(HH ensured that there was just one number absent from 1 to M and any two intervals didn’t overlap)

Input

Multiple cases ended with 2 zeros.

N M (0<N<=10^7, 0<M<=10^8, integer)

N lines followed, each containing two integers:

xi yi  (integer)

Output

An integer (the absent number)

Sample Input
3 10
3 5
1 2
7 10
2 5
1 2
4 5
0 0
Sample Output
6
3
Hint

since there is too much data, scanf  is recommended.

If you need to use 64bit integer, the following code may be helpful:

#include<iostream>
using namespace std;

int main()
{
    long long a = 12345678987654321LL;
    int b = 88888888, c = 99999999;
    cout << a << endl << (long long)b * c << endl;
    return 0;
}

Source
He Jian, HUST Campus 2010, Preliminary