import sys
N = int(raw_input())

v = []

for i in range(N):
    a, b = map(int, raw_input().split(' '))
    v.append(int(a) + 4 * int(b))

r = max(v)

ans = sum((r - x) // 2 for x in v)

ok = all((r - x) % 2 == 0 for x in v)

if ok:
    print(ans)
else:
    print(-1)