The sum part of the formula for a signals power $$P_x=\lim_{M\rightarrow \infty }\frac{1}{2M+1}\sum_{n=-M}^{M}\left | x[n] \right |^2$$

I have used this formula to find out the powers for different signals, but the sum part of the formula still puzzles me. 
Lets take one example:
$$x[n] = 4i+1$$ 
Simply calculating the absolute value, I got this:
$$|x[n]|^2=|4i+1|^2=(\sqrt{4^2+1^2})^2=17$$
17 seems to be the answer I got after going through the rest of the formula, but I am not sure how the sum works. By using Wolframa, I arrived at:
$$\sum_{n=-M}^{M}17 =34M+17$$
Which would complete the limit part of the formula with the final result of 17, as discussed. How does this sum function, as described above?
A more general expression states that for $ M \geq N$:

$$ \sum_{n= N}^{n = M} c =  (M-N+1) \cdot c $$

where the derivation simply relies on fact that the epxression has (M-N+1) terms :

$$ \sum_{n= N}^{n = M} c = \{ c + c + ... + c\} = (M-N+1) \cdot c $$

And when applied for your particular case (with $N = -M$) it becomes:
$$ \sum_{n= -M}^{n = M} c = (M-(-M)+1) \cdot c = (2M+1) \cdot c $$