http://poj.org/problem?id=3198
/* @author: */
import java.math.BigInteger;
import java.util.*;
public class Main
{
public static BigInteger get( BigInteger h ) {
BigInteger s = h.shiftLeft(1);
BigInteger a = BigInteger.ONE.shiftLeft( s.bitLength()/2-1 ).subtract( BigInteger.ONE );
BigInteger b = BigInteger.ONE.shiftLeft( s.bitLength()/2+1 ), c;
while( a.add(BigInteger.ONE).compareTo(b) < 0 ) {
c = a.add( b ).shiftRight(1);
if( c.multiply( c.add(BigInteger.ONE)).compareTo( s ) <=0 )
a = c;
else
b = c;
}
return a;
}
public static BigInteger X, Y;
public static void Get( BigInteger s ) {
BigInteger t = get( s );
Y = s.subtract( t.multiply(t.add(BigInteger.ONE)).shiftRight(1) );
X = t.subtract( Y );
}
public static void main(String args[])
{
Scanner cin = new Scanner(System.in);
String s;
while( true ) {
s = cin.next();
if( s.equals( "*" ) )
break;
BigInteger a = new BigInteger( s ), t = BigInteger.ONE, h = BigInteger.ONE, sum = BigInteger.ZERO,
x =BigInteger.ONE,
y=BigInteger.ONE, xt=BigInteger.ONE, yt=BigInteger.ONE, x0 = BigInteger.ONE, y0=BigInteger.ONE;
Get( a );
t = X; h = Y;
//System.out.println( X+","+Y );
//if(X==t)continue;
int n = t.intValue();
for( int i=0; i< n; i++ ) {
a = h;
if( i < n-1 ) {
Get( a );
t = X; h = Y;
}
else
t = a;
Get( t );
x = X; y = Y;
if( i == 0 ) {
x0 = x; y0 = y;
}
else {
sum = sum.add( x.multiply(yt).subtract( y.multiply(xt) ));
}
xt = x; yt = y;
//System.out.println( x + "," + y );
}
sum = sum.add( x0.multiply(yt).subtract( y0.multiply(xt) ));
sum = sum.abs();
System.out.print( sum.shiftRight(1) );
if( sum.equals(sum.shiftRight(1).shiftLeft(1)))
System.out.println( ".0" );
else
System.out.println( ".5");
}
return;
}
}