The Priest Mathematician

http://acm.hust.edu.cn/vjudge/contest/view.action?cid=31329#problem/F

f[0] = 1 , f[ i ] = f[ i - 1 ] + 2 ^( n - k - 2 )

import java.util.Scanner;
import java.math.BigInteger ;
public class Main
{
	public static void main(String[] args )
	{
		Scanner cin = new Scanner( System.in );
		BigInteger[] f = new BigInteger[ 10005 ] ;
		int k = 1 , cnt = 0 ;
		f[ 0 ] = new BigInteger( "0" ) ;
		BigInteger d = new BigInteger( "1" ) ;
		for( int i = 1;  i <= 10004 ; ++i )
		{
			f[ i ] = f[ i - 1 ].add( d ) ;
			cnt++;
			if( cnt == k )
			{
				cnt = 0 ;
				k++;
				d = d.multiply(BigInteger.valueOf(2) ) ;
			}
		}
		while( cin.hasNext())
		{
			int n = cin.nextInt();
			System.out.println( f[ n ] ) ;
		}
	}
}