You wouldn't add 1, but multiply. First multiply all factors with the same base. When you do that, their exponents get added:
[tex]C^0D^{-5}D^6C^1C^2C^3=C^{0+1+2+3}D^{-5+6}=C^6D^1[/tex]
Notice how [tex]C^0C^1=C^{0+1}=C^1[/tex]. The factor with zero exponent contributes nothing new to the product, so the effect is the same as multiply [tex]C^1[/tex] by 1. In general, [tex]a^0=0[/tex] for any non-zero number [tex]a[/tex].
