N=int(input())
AB=[tuple(map(int,input().split())) for i in range(N)]

X=[]

for a,b in AB:
    X.append(a+b*4)

O=[0,0]

for x in X:
    O[x%2]+=1

if O[0]>=1 and O[1]>=1:
    print(-1)
    exit()

MAX=max(X)

ANS=0
for x in X:
    ANS+=MAX-x

print(ANS//2)