import sys
N = input()
space_max = 0
d = [0] * (10**3)
for i in range(N):
    a,b = map(int, raw_input().split())
    d[i] = a + 4 * b
    space_max = max(space_max, a + 4 * b)
ans = 0
for i in range(N):
    rest = abs(space_max - d[i])
    if (rest % 2 != 0):
        print -1
        sys.exit()
    ans += (rest) / 2
print ans