N = int(input())
ab = [list(map(int, input().split())) for i in range(N)]
W = [ab[i][0] + 4 * ab[i][1] for i in range(N)]
cnte = 0
cnto = 0
for i in range(N):
    if W[i] % 2:
        cnto += 1
    else:
        cnte += 1
if cnto * cnte > 0:
    print(-1)
    exit()
W.sort(reverse = True)
maxx = W[0]
ans = 0
for i in range(1, N):
    ans += (W[0] - W[i]) // 2
print(ans)