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