Multiples of seven

For every natural $n$ , let $X(n)$ be the smallest natural $m$ such that $m$ ends with $n$ and $m$ is a multiple of 7. For instance, $X(1) = 21$ , $X(2) = 42$ , $X(3) = 63$ , …, $X(7) = 7$ , $X(8) = 28$ , $X(9) = 49$ , $X(10) = 210$ , $X(11) = 511$ , … Let $S$ be the infinite concatenation of $X(i)$ for every $i \ge 1$ , that is, $S = 21426314355672849210511...$ . Which is the $i$ -th digit of $S$ ?

Input

Input consists of several cases, each with a natural $i$ between 1 and $10^{15}$ .

Output

For every $i$ , print the $i$ -th digit of $S$ (starting at one).

Problem information

Author: Salvador Roura

Generation: 2026-01-25T11:55:25.375Z