Fix mistake in the formula that simplifies to (a*d + b*c)
Fix mistake in the formula that simplifies to (a*d + b*c)

(a+b)*(c+d) - a*c - b*c  = (a*c + a*d + b*c + b*d) - a*c - b*c

should be

(a+b)*(c+d) - a*c - b*d  = (a*c + a*d + b*c + b*d) - a*c - b*d
jawwad/swift-algorithm-club,boqian2000/swift-algorithm-club,uteschiehlen/swift-algorithm-club,tkester/swift-algorithm-club,raywenderlich/swift-algorithm-club,eugenepavlyuk/swift-algorithm-club,boqian2000/swift-algorithm-club,jawwad/swift-algorithm-club,tomtclai/swift-algorithm-club,hollance/swift-algorithm-club,tomtclai/swift-algorithm-club,jawwad/swift-algorithm-club,daisysomus/swift-algorithm-club,aatalyk/swift-algorithm-club,TheIronBorn/swift-algorithm-club,aatalyk/swift-algorithm-club,TheIronBorn/swift-algorithm-club,tkester/swift-algorithm-club,daisysomus/swift-algorithm-club,raywenderlich/swift-algorithm-club,uteschiehlen/swift-algorithm-club,eugenepavlyuk/swift-algorithm-club,raywenderlich/swift-algorithm-club