n=int(input())
l=[]
for i in range(n):
    a,b=map(int,input().split())
    l.append(a+4*b)

x,y=0,0
for c in l:
    if c%2==0:
        x+=1
    else:
        y+=1
if (x==0 and y==n) or (x==n and y==0):
    ok=1
else:
    ok=0


if ok==0:
    print(-1)
    exit()


m=max(l)
ans=0
for x in l:

    ans+=abs(x-m)//2

print(ans)