Keep shooting until you win!

Suppose Lucy and Peter shoot independently until each shoots each other perfectly.They have probabilities $\frac{2}{5}$ and $\frac{3}{7}$ of shooting each other perfectly at each shot respectively.Find the probability that Peter will require more shots than Lucy.

The answer is in the form $\frac{a}{b}$ where a and b are co-prime.

Find a+b .