Find the absent number
Time Limit : 10 Second
Memory Limit : 128 MB
Submission: 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