N = int(input())
x = [list(map(int, input().split())) for _ in range(N)]
x = list(map(lambda x: x[0]+x[1]*4, x))
if len(set(map(lambda x: x&1, x)))==2:
    print(-1)
    exit()
lim = max(x)
ans = 0
for y in x:
    ans += (lim-y)//2
print(ans)