n = int(input())
base = []
for _ in range(n):
    a, b = map(int, input().split())
    base_i = a + 4 * b
    base.append(base_i)

# Check if all base_i have the same parity
par = base[0] % 2
possible = True
for num in base[1:]:
    if num % 2 != par:
        possible = False
        break

if not possible:
    print(-1)
else:
    max_base = max(base)
    total = 0
    for num in base:
        diff = max_base - num
        total += diff // 2
    print(total)