Once upon a time Professor Idioticideasinventor was travelling by train. Watching cheerless landscape outside the window, he decided to invent the theme of his ...

In the English language, nouns are inflected by grammatical number - that is singular or plural. In this problem we use a simple model of constructing plural fr...

For given integer N (1<=N<=10^4) find amount of positive numbers not greater than N that coprime with N. Let us call two positive integers (say, A and B, for ex...

Mary is a teacher in a middle school and she has a table seat storing students’ names and their corresponding seat ids.

There are two kinds of threads, oxygen and hydrogen. Your goal is to group these threads to form water molecules. There is a barrier where each thread has to wa...

Let f(n) be a sum of digits for positive integer n. If f(n) is one-digit number then it is a digital root for n and otherwise digital root of n is equal to digi...

There is an equation ax + by + c = 0. Given a,b,c,x1,x2,y1,y2 you must determine, how many integer roots of this equation are satisfy to the following condition...

For given number N you must output amount of N-digit numbers, such, that last digits of their square is equal to 987654321.

It is not easy to prepare a problem for a programming contest. Petya and Vasya decided that problem “A+B” needs at least n distinct solutions to be written. It ...

First year of new millenium is gone away. In commemoration of it write a program that finds the name of the day of the week for any date in 2001.