1 and 2, 2 and 3, 3 and 4 and so on they have a path difference of 1/100 of the wavelength .. and this should be in the range of the wavelength itself .. moreover number of such pairs are also higher than the ones causing destructive superpositioning .. still, we see a dark spot?
If u think in such a way then explanation would be 1 plus 2 and 3 plus 4 will be cancelled out with 98 plus 99 and 99 plus 100.... Both ways it's the same.