The Garman Kohlhagen (1983) closed form currency option pricing model is defined as follows:

OV = option value.

S =current exchange rate.

X = exercise rate (strike).

r = domestic country's risk-free interest rate, expressed with continuous compounding.

r_f = foreign country's risk-free interest rate, expressed with continuous compounding.

vol = volatility of the relative price change of the underlying asset.

T = time to maturity measured in years (actual/365 basis).

N(.) = cumulative normal distribution of (.).

iPC = 1 for call / -1 for put.