(2+4+6+8+.+98+100)-(1+3+5+7+.+97+99)
=2+4+6+8+.+98+100-1-3-5-7-.-97-99
=2-1+4-3+6-5+8-7+.+98-97+100-99
=1+1+1+1+...+1+1
=1*50
=50