n = int(input())
A = []
ans = 0
for i in range(n):
    a, b = map(int,input().split())
    A.append(a + 4 * b) 
m = max(A)
for i in A:
    if (m - i)&1==0:
        ans += (m - i) // 2
    else:
        ans = -1
        break
print(ans)