n=int(input())
ab=[tuple(map(int,input().split())) for _ in range(n)]
x=0
for i,j in ab:
    x=max(x,i+j*4)
ans=0
for i,j in ab:
    if (x-i)&1:
        print(-1)
        exit(0)
    ans+=x-i-j*4>>1
print(ans)