N = int(input())
D = [list(map(int, input().split())) for x in range(N)]
L = []
for n in range(N):
    L.append(D[n][0]+D[n][1]*4)

m = max(L)
S = 0
n = 0

while n < N:
    if (L[n-1] + L[n]) % 2 == 0:
        S += int((m - L[n])/2)
        n += 1
    else:
        S = -1
        break

print(S)