How many proofs of 1=0 are there?

Step 1) Let $a,b,c,d$ be non-zero numbers that exist in the complex set of numbers. Let these numbers also satisfy the equation $a+b=cd$

Step 2) We can divide both sides by $c$ : $a+b=cd \Longrightarrow \frac{a+b}{c}=d$

Step 3) We can subtract $d$ from both sides: $\frac{a+b}{c}=d \Longrightarrow \frac{a+b}{c}-d=0$

Step 4) We can divide both sides by $\cfrac{a+b}{c}-d$ : $\frac{a+b}{c}-d=0 \Longrightarrow \frac{\frac{a+b}{c}-d}{\frac{a+b}{c}-d}=\frac{0}{\frac{a+b}{c}-d}$ Since any number divided by itself is equal to $\blue{1}$ and $0$ divided by any number is equal to $\orange{0}$ , we can conclude that $\green{ 1 = 0}$ : $\frac{\blue{\frac{a+b}{c}-d}}{\blue{\frac{a+b}{c}-d}}=\frac{\orange{0}}{\orange{\frac{a+b}{c}-d}} \Longrightarrow \green{1 = 0} \; \; \; \square$

Of course this proof is $100\%$ valid, but try to find any flaws. If you find one, what step is it?