Subtract Leading Digit

Vasya learned about integer subtraction in school. He is still not very good at it, so he is only able to subtract any single digit number from any other number (which is not necessarily single digit). For practice, Vasya chose a positive integer $n$ and wrote it on the first line in his notepad. After that, on the second line he wrote the result of subtraction of the first digit of $n$ from itse