# No.178 美しいWhitespace (1)
N = int(input())
a = []
b = []

for i in range(N):
    m, n = [int(j) for j in input().split()]
    a.append(m)
    b.append(n)

lengths = [a[i] + 4 * b[i] for i in range(N)]
len_max = max(lengths)

whitespace = 0

for i in range(N):
    if (len_max - (a[i] + 4 * b[i])) % 2 == 1:
        whitespace = -1
        break
    else:
        whitespace += (len_max - (a[i] + 4 * b[i])) // 2
    
print(whitespace)