#coding:utf-8
	
def main():

	N=input()
	n=[raw_input().split() for i in range(N)]

	s=[]
	for i in n:
		s.append(map(int,i))

	t=[]
	for a,b in s:
		t.append(a+b*4)
	
	cnt=0
	if len(t)==1:
		return 0
	else:
		t=sorted(t,reverse=True)
		for i in t[1:]:
			if (t[0]-i)%2==0:
				cnt+=(t[0]-i)/2
			else:
				return -1
		
		return cnt
		
		
if __name__=='__main__':
	print main()