At first it was confusing but I later figure out what the question is asking

for example if the input is 'CDA', and the values are 3,4,1

the index of each char in the string in reverse order plus one is 2, 1, 0

the formula is

result = value(char) * 26^(order(char))

so in this case.

result = 3 * (26^2) + 4 * (26^1) + 1 * 26^0 = 2133

This makes sense to me and hope my explanation is helpful

```
class Solution(object):
def titleToNumber(self, s):
res = 0
for i in range(len(s)):
coef = ord(s[-1-i])-64
value = 26**i
res += (coef*value)
return res
```