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]
ans = 0
for i in xrange(N):
	if m&1 != l[i]&1:
		print -1
		break
	ans += m-l[i]
else:
	print ans/2