((2*2+4*4+…+100*100)-(1*1+3*3+...99*99))/(1+2+...+10+9+8...+1)
=((2*2-1*1)+(4*4-3*3)+(6*6-5*5)+...+(100*100-99*99))/((1+9)+(2+8)+...+(9+1)+10)
=((2+1)+(4+3)+(6+5)+...+(100+99))/(10*10)
=5050/100
=50.5