n = int(input())
T = []
for i in range(n):
    a,b = map(int, input().split())
    T.append(a+b*4)
T.sort(reverse=True)
ans = 0
for t in T:
    if (T[0]-t)%2 != 0:
        print(-1)
        exit()
    else:
        ans += (T[0]-t)//2
print(ans)