N = int(raw_input()) f = lambda x:int(x[0])+int(x[1])*4 l = [f(raw_input().split()) for i in xrange(N)] m = max(l) 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))