N = int(input())
A, B = [0] * N, [0] * N
ma = 0
for i in range(N):
    A[i], B[i] = map(int, input().split())
    ma = max(ma, A[i] + 4 * B[i])
    
ans = 0
for i in range(N):
    v = A[i] + 4 * B[i]
    if (ma - v) % 2:
        print(-1)
        exit()
    ans += ma - v
    
print(ans // 2)