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

p = max(L)
ans = 0
for i in range(len(L)) :
    L[i] = p-L[i]
    if L[i] % 2 != 0 :
        print(-1)
        break
else :
    print(sum(L) // 2)