N = int(raw_input())
f = lambda x:int(x[0])+int(x[1])*4
l = sorted(f(raw_input().split(" ")) for i in xrange(N))
m = l[-1]
if len(filter(lambda x:(m&1)==(x&1),l)) < N:
	print -1
else:
	print sum((m-l[i])/2 for i in xrange(N))