N=int(input())
s=[0]*N
ans=0
for i in range(N):
    a,b=map(int,input().split())
    s[i]=a+4*b
for k in s:
    if (max(s)-k)%2==0:
        ans+=(max(s)-k)//2
    else:
        print(-1);exit()        
print (ans)