Using a formula filed in a formula field

I am working on a crystal report in which I've to make use of the values from two formula fields to make anew formula filed that is being used in my details section.

I have tried it out the below mentioned formula earlier, but some how m not able to get the desired result.

if not isnull ({@C_FEB} and not isnull{@E_FEB}) )

then cstr({@C_FEB} & ':' & {@E_FEB})

else

case (isnull({@C_FEB}))

then {@E_FEB}

else

case (isnull({@E_FEB}))

then {@C_FEB}

the new formula field is final_feb

What I wanted this formula to do is

If c_feb is not null and e_feb is not null then output should be the concatenation of the two values from c_feb and e_feb as c-feb:e_feb

If c_feb is null then the value of final_feb should be e_feb.

If e_feb is null then the value of final_feb should be c_feb.

if the value for both the formula fields is null then nothing shoulh get displayed.

Earlier I was trying only

cstr({@C_FEB} & ':' & {@E_FEB}) where I couldn't get rid of colon in any case. How ever, the formula did not work when i tried using the if-else statement.

Can anyone please suggest any better way of doing it and point out where exactly I'm going wrong.

p.s-

I'm just a beginner in Crystal Reports so please bear with my limited knowledge and incorrect syntax usage.