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

s = max(rec)
num = 0
for i in range(n):
    num += s - rec[i]

if num % 2 == 0:
    print(num // 2)
else:
    print(-1)