题目描述Little X has met the following problem recently. Let’s define $f(x)$ as the sum of digits in decimal representation of number $x$ (for example, $f(1234)=1+2+3+4$). You are to calculate $\sum_

题目描述In the country of Never, there are $n$ cities and a well-developed road system. There is exactly one bidirectional road between every pair of cities, thus, there are as many as ${n(n - 1) \over 2}

Goodbye 2017, Hello 2018!

题目描述Two of the three members of the winning team of one of the ACM regional contests are going to meet in order to train for the upcoming World Finals. They decided that they will meet sometime betwee

题目描述The Queen of Byteland is very loved by her people. In order to show her their love, the Bytelanders have decided to conquer a new country which will be named according to the queen’s name. This ne

题目描述Joe is a frog who likes to jump a lot. In fact, that’s all he does: he jumps forwards and backwards on the integer axis (a straight line on which all the integer numbers, both positive and negativ

题目描述A sequence $A$ is called an integer sequence of length $N$ if all its elements $A_1, A_2, \ldots, A_N$ are non-negative integers less than $2000000000$. Consider two integer sequences of length $N

题目描述$N$ friends gathered in order to play chess, according to the following rules. In the first game, two of the $N$ friends will play. In the second game, the winner of the first game will play again

题目描述Let’s consider a string of non-negative integers, containing $N$ elements. Suppose these elements are $S_1, S_2, \ldots, S_N$, in the order in which they are placed inside the string. Such a strin

题目描述Little Johnny painted on a sheet of paper a polygon with $N$ vertices. Then, for every edge of the polygon, he drew the middle point of the edge. After that, he went to school. When he came back,