This function receives as input one string containing digits, letters or special symbols. 

The function should return one float number contaning the average calculated considering all the digits in the string. If there are no digits the function should return 0.0.

An example: `avgDigs('123a456')` should return `3.5`

So far I have 

```
def avgDigs (st):  
    count = 0  
    total = 0  
    for i in st:  
        if st.isdigit():  
            count += int(i)  
            total += 1  
    print float(count)/total

```

How can I ignore letters and only add the digits?