n = int(input())
c=list()
for i in range(n):
    a1,b1=map(int,input().split())
    c.append(a1+4*(b1))

d = [max(c) - t for t in c]

for e in d:
    if e%2 == 1:
        print(-1)
        break
else:
    print(int(sum(d)/2))