Calculating a field value based on 2 other fields

Calculating a field value based on 2 other fields

I have put below into the workflow for a subform but its not giving the right result. Approximate cost field needs to be calculated based on Lump cost and Number of airstrips. Please assist.

if(Number_of_airstrip != null)
{
input.Approximate_Cost = input.Lump_Cost / input.Number_of_airstrip;
}