n = int(input())
ab = [list(map(int, input().split())) for _ in range(n)]

max_space = 0
c = []
for a, b in ab:
    c.append(a + b * 4)
    max_space = max(max_space, a + b * 4)
    
ans = 0
for cc in c:
    if (cc - max_space) % 2 == 1:
        print(-1)
        exit()
    ans += (max_space - cc) // 2
    
print(ans)