Given a string of integers, return the number of odd-numbered substrings that can be formed. 
For example, in the case of `"1341"`, they are `1, 1, 3, 13, 41, 341, 1341`, a total of `7` numbers. 
`solve("1341") = 7`.  See test cases for more examples.
Good luck!
If you like substring Katas, please try 
[Longest vowel chain](https://www.codewars.com/kata/59c5f4e9d751df43cf000035)
[Alphabet symmetry](https://www.codewars.com/kata/59d9ff9f7905dfeed50000b0)
def solve(s):
    return sum(i for i, x in enumerate(s, 1) if x in '13579')
def solve(s):
    return sum(i for i, x in enumerate(s, 1) if x in '13579')