Looping ASCII characters and decrypting the message

My teacher gave me a homework: -

You are given a string (of at most 100 characters) consisting of lowercase characters, terminated with a # symbol. The string is the "encrypted" form of an original string as follows - each character in the original string has been shifted by a fixed integer n (where 1<= n <= 25).

Assuming that a is the 0th character in the alphabet, ..., and z is the 25th character of the alphabet, in the encrypted version, we have:

'a' becomes the nth character in the alphabet, 'b' becomes the (n+1)%26th character in the alphabet, ...

and so on.

n is not known to you. You only know that the first character represents 't'. From this information, you have to output the original string.

Here is my code for it: -

#include <stdio.h>
#include <stdlib.h>

int main()
{
    char array[100]; //string will be stored here.
    char a;
    int i = 0; //initializer
    
    scanf("%c", &a);
    
    int counter = 0; // to count the size of the string
    
    while(a != '#')
    {
        array[i] = a;
        i++;
        counter++;
        scanf("%c", &a);
    }
    
    a = 't';
    
    int temp = a - array[0]; //To get the offset of the encrypted message.
    
    for(i = 0; i < counter; i++)
    {
        if((array[i] + temp) >= 'a' && (array[i] + temp) <= 'z')
        {
            array[i] = array[i] + temp; //Applying the offset to the encrypted message to get the true message.
        }
        
        else if((array[i] + temp) < 'a')
        {
            array[i] = 'z' - (temp - (array[i] - 'a')); //If offset is large and it falls below ASCII charater number 'a' then loop back to 'z'.
        }
        
        else
        {
            array[i] = 'a' + (temp - ('z' - array[i])); //If offset is large and it falls above ASCII charater number 'z' then loop back to 'a'.
        }
    }
    
    puts(array);
    
    return 0;
}

Now the logic which I have applied is certainly wrong. All of my test cases are failing. I suspect the problem is in the following snippet of the code: -

for(i = 0; i < counter; i++)
    {
        if((array[i] + temp) >= 'a' && (array[i] + temp) <= 'z')
        {
            array[i] = array[i] + temp; //Applying the offset to the encrypted message to get the true message.
        }
        
        else if((array[i] + temp) < 'a')
        {
            array[i] = 'z' - (temp - (array[i] - 'a')); //If offset is large and it falls below ASCII charater number 'a' then loop back to 'z'.
        }
        
        else
        {
            array[i] = 'a' + (temp - ('z' - array[i])); //If offset is large and it falls above ASCII charater number 'z' then loop back to 'a'.
        }
    }

I am not able to do this easy question. Please help.

Some of the test cases are mentioned below: -