Each letter of the english alphabet is corresponding to an integer number.

a = 1, b = 2, ... , z = 26

For a given string, calculate the string's number.  Example: "abc" = 1+2+3 = 6        "ad" = 1 + 4 = 5

Now use this value to indentify a letter, for example: "ad" = 1 + 4 = 5 = e (5 is the corresponding number of e)

If this value is greater than 26, devide it with 26 and keep the integer part. For example if the value is 38 then 38 / 26 = int(1,46) = 1 = a

For example you have two strings: "abc" = 1+2+3 = 6 = f        "ad" = 1 + 4 = 5 = e  , then the hidden message is "fe". So each string is actually a letter of the hidden message.