Here is the horrendous thing:
N=5+15+2+5−2+3−22=k−m.
We will deal with the two terms separately. The first term, which we call k, we will square:
k2====(5+15+2+5−2)25+1)(5+2)+2(5+2)(5−2)+(5−2)5+125+25−45+125+2=2.
So k=2. The second term, m does not require squaring:
m=3−22=2−22+1=(2)2−22+1=(2−1)2=2−1.
Thus N=k−m=1. Surprise!