Canadian Computing Competition: 2009 Stage 1, Junior #1

The International Standard Book Number (ISBN) is a 13-digit code for identifying books. These numbers have a special property for detecting whether the number was written correctly.

The 1-3-sum of a 13-digit number is calculated by multiplying the digits alternately by 1's and 3's (see example) and then adding the results. For example, to compute the 1-3-sum of the number 9780921418948 we perform:

\[\displaystyle 9 \times 1 + 7 \times 3 + 8 \times 1 + 0 \times 3 + 9 \times 1 + 2 \times 3 + 1 \times 1 + 4 \times 3 + 1 \times 1 + 8 \times 3 + 9 \times 1 + 4 \times 3 + 8 \times 1 = 120\]

The special property of an ISBN is that its 1-3-sum is always a multiple of \(10\) .

Write a program to compute the 1-3-sum of a 13-digit number. To reduce the amount of typing, you may assume that the first ten digits will always be 9780921418 , like the example above. Your program should input the last three digits and then print its 1-3-sum. Use a format similar to the samples below.

Sample Input 1

9
4
8

Sample Output 1

The 1-3-sum is 120

Sample Input 2

0
5
2

Sample Output 2

The 1-3-sum is 108