=(10+9)*(10-9)+(8+7)*(8-7)+(6+5)*(6-5)+(4+3)*(4-3)+(2+1)*(2-1)
=(10+9)*1+(8+7)*1+(6+5)*1+(4+3)*1+(2+1)*1
=10+9+8+7+6+5+4+3+2+1
=55