Invalid type argument of unary '*' (have int?

Question

I have read other questions like this but none seemed to work... My code is:

int flowRateFormula(int pipeDiameter,double velocity)
{
int integer3;

integer3=PI*(1/4)*(pow(pipeDiameter,2))*velocity;

return integer3;

 }

And the error is:

flowRate.c: In function ‘flowRateFormula’:
flowRate.c:38:13: error: invalid type argument of unary ‘*’ (have ‘int’)

What to do? BTW PI IS DEFINED