def f(q):
    return q[0]+4*q[1]
w=[f(list(map(int,input().split()))) for i in range(int(input()))]
k=set([s%2 for s in w])
if len(set([s%2 for s in w]))==2:
    print(-1)
else:
    m=max(w)
    print(sum([abs(w[i]-m)//2 for i in range(len(w))]))