N=int(raw_input())
ab=[map(int,raw_input().split()) for i in range(N)]
s=0
for i in range(N):
    s=max(s,ab[i][0]+4*ab[i][1])
ans=0
for i in range(N):
    if ans<0:
        continue
    if (ab[i][0]+4*ab[i][1])%2!=s%2:
        ans=-1
    else:
        ans+=(s-ab[i][0]-4*ab[i][1])/2
print ans