You are consuming a line at, which is discarded
while((str=input.readLine())!=null && str.length()!=0)
and reading a bigint at
BigInteger n = new BigInteger(input.readLine());
so try getting the bigint from string which is read as
BigInteger n = new BigInteger(str);
Constructor used: BigInteger(String val)
Aslo change while((str=input.readLine())!=null && str.length()!=0) to
while((str=input.readLine())!=null)
see related post string to bigint
readLine()
Returns:
A String containing the contents of the line, not including any line-termination characters, or null if the end of the stream has been reached
see javadocs