adobe livecycle pdf form

hi,

i have a pdf form made by adobe livecycle designer. here there r 2 fields. say one field A and anothe is B . i want to populate info in field b which will depend on value of A. IF i put 4 in A then B will 17., if A 5 B 21 .... etc. can u pls help to make such tye of code?

thanks

Ali