#Bash + GNU utilities, 32

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    rev|fold -1|nl -s*|paste -sd+|bc

Input read from STDIN.

[Try it online][1].

###Explanation

    rev                              # reverse digits
       |fold -1                      # split each digit onto its own line
               |nl -s*               # prefix line numbers; separate with "*" operator
                      |paste -sd+    # join lines onto one line, separated with "+" operator
                                 |bc # arithmetically evaluate

  [1]: https://tio.run/##S0oszvj/vyi1rCYtPydFQdewJi9HQbdYq6YgsbgkFchK0a5JSv7/39TY3NAMAA