How to call existing function within a custom function

How to call existing function within a custom function

Hello,

I'm using the script below to create a custom function but it gives the error "not able to find PV function."

How do I call the PV function?

float PVD(float rate,float n,float d,float pmt,float fv,int type)
{
PVD = PV(rate,n+d,pmt,fv,type)-PV(rate,d,pmt,fv,type);
return PVD;

 }