#coding=UTF-8

N=int(input())
mojiren=list(range(0,N,1))

for idx in range(0,N,1):
    mojir=input()
    hyo=mojir.split(" ")
    mojiren[idx]=int(hyo[0])+4*int(hyo[1])

ma=max(mojiren)
ans=0
for idx in range(0,N,1):
    if (ma-mojiren[idx])%2==0:
        ans=ans+(ma-mojiren[idx])//2
    else:
        ans=-1
        break
print(ans)