n = int(input())

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

oe = []
for i in ls:
    if i%2 == 0:
        oe.append(0)
    else:
        oe.append(1)

if len(set(oe)) == 2:
    print('-1')
else:
    total = 0
    mx = max(ls)
    for i in ls:
        total += mx - i 
    print(int(total/2))