if n=2 S={HH,HT,TH,TT} difference is 0 ,2 so n-3 ie -1 not possible
if n=3 S={HHH HHT HTH HTT THH THT TTH TTT}
here difference is 3,1, and n-3 =0 not possible
for n= even n-3 is odd and difference between head and tail will be always even
for n= odd n-3 is even and difference between head and tail will always odd
so probability=0