adding return values from different sub report

[ummekulsum4321]

i have 4 sub report which returns thier summed up values

for example A!,b1 c1,d1 are the variable of main report which are used to return the values from sub report all are of type double

i have to add a1+c1

and b1+d1

suppose eany one of the value is null the addiition doesnt happen

what i want is if the return value is null i want it to take it as 0 and add up how can i achive that