2 sec, 256 M
A lesson of the philanthropy has the next structure: professor Mitrofan Feofanovich Cobblestone asks students absolutely randomly one by one until he asks somebody twice (only this lesson history considered). The lections course finishes when all the students are asked at least once on some lesson. You are interested in the expectation of the lessons’ number.
Input. The input line contains a natural number N (1 ≤ N ≤ 100) – the number of students.
Output. Print the expectation of lessons’ number with the accuracy up to 9 digits after the decimal point.
Idea – Oleg Alexandrovich Petrov
KBIS Software Developer