100x100-99x99+98x98-97x97+.+4x4-x4-3x3+2x2-1=(100+99)(100-99)+(98+97)(98-97)+.+(4+3)(4-3)+(2+1)(2-1)=199+195+.7+3=(199+3)+(195+7)+(191+11)+(187+15)+.+(103+99)=5050