N = int(input())

S = []

for _ in range(N):
    a,b = map(int,input().split())
    S.append(a+b*4)

M = max(S)
cnt = 0

for s in S:
    if (M-s)%2 == 1:
        print(-1)
        exit()
    cnt += (M-s)//2

print(cnt)