#[Japt](https://github.com/ETHproductions/japt) [`-m`](https://codegolf.meta.stackexchange.com/a/14339/), 13 [bytes](https://en.wikipedia.org/wiki/ISO/IEC_8859-1)

    ;o ?UÉ:Cg8+Uc

[Try it](https://petershaggynoble.github.io/Japt-Interpreter/?v=1.4.6&flags=LW0&code=O28gP1XJOkNnOCtVYw&input=Wy0xLCAwLCAyNTcsICJ4Il0)

##Explanation:
    ;o ?UÉ:Cg8+Uc
    -m              // Map U through the input:
     o              // Try to create a range [1...U]
       ?            //   If that creates a range (number):
        UÉ          //     Return U-1
          :         //   Else, return:
    ;      C        //     alphabet (a...z)
            g       //     Index:
             8+Uc   //       8 + char-code of U

**Note**: `;` turns `C` into the lowercase alphabet