N = int(input())
odd_flg = 0
even_flg = 0
flg = 0
max = 0
count = 0

for i in range(N):
    a, b = map(int, input().split())
    ck = a + (4 * b)
    count += ck

    if ck % 2 == 0:
        even_flg = 1
    else:
        odd_flg = 1

    if even_flg == odd_flg == 1:
        print(-1)
        flg = 1
        break
    
    if max < ck:
        max = ck
    
if flg == 0:
    print(int(((N * max) - count) / 2))