n = int(raw_input())
l = []
for _ in range(n):
    a,b = map(int, raw_input().split())
    l.append(a+b*4)
m = max(l)
c = 0
for i in l:
    if (m - i) % 2 == 1:
        print -1
        break
    c += (m-i) / 2
else:
    print c