Deriving Chi-Square for contingency table 2x2

Given a 2x2 contingency table, with a b c d as the observed values and n as the total, I have to show from the fact that Chi-Square = Sum(((O-E)^2)/E) Where E is the expected value of the observed, that:
Chi-Square= (n(ad-bc)^2)/(a+b)(a+c)(b+d)(c+d).
I just cant seem to get there. Any links or tips about how this formula is derived?
Thanks in advance