#Japt -m, 13 12 bytes

-1 byte thanks to Shaggy

;¤?UÉ:Cg8+Uc

Try it

##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

Japt -m, 13 12 bytes

-1 byte thanks to Shaggy

;¤?UÉ:Cg8+Uc

Try it

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

#Japt -m, 13 bytes

;o ?UÉ:Cg8+Uc

Try it

##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

#Japt -m, 13 12 bytes

-1 byte thanks to Shaggy

;¤?UÉ:Cg8+Uc

Try it

##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

#Japt -m, 13 bytes

;o ?UÉ:Cg8+Uc

Try it

#Japt -m, 13 bytes

;o ?UÉ:Cg8+Uc

Try it

##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