contest math - Find all functions $mathbb{R}^{+}rightarrow mathbb{R}$

Find all functions $f$: $\mathbb{R}^{+}\rightarrow \mathbb{R}$ such that

$$f\left ( \frac{x}{y} \right )= f(x)+f(y)-f(x)f(y)$$

for all $x,y\in\mathbb{R}^{+}$. Here, $\mathbb{R}^{+}$, denotes the set

of all positive real numbers.

I really couldn't solve it. Any help?

This question from IMO Competition.