HDU - 5066 Harry And Physical Teacher ( 物理-动量守恒+能量守恒 )

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <set>
#include <stack>
#include <list>
#include <vector>
#include <map>
#include <ctime>
#include <cmath>
using namespace std;
typedef __int64 LL;
#define CLR( a, b )     memset( a, b, sizeof(a) );

void Orz()
{
    LL v1,v2;
    while( ~scanf( "%I64d %I64d", &v1, &v2) )
    {
        printf( "%I64d\n", 2*v1 - v2 );
    }
}

int main()
{
    Orz();
    return 0;
}