12. Write $0, 1, 2, 3, 4, 5, 6, 7$ on the eight vertices of a cube (each vertex with one number, and all numbers can only be used once), such that the sum of the two numbers on each edge is a prime number. Then the maximum sum of the four numbers on one face is $\qquad$.
【Analysis】To ensure that the sum of the two numbers on each edge is a prime number, the two adjacent numbers must be one odd and one even. First, determine 0, which must be adjacent to $3, 5, 7$. The remaining $2, 4, 6$ can be enumerated. As shown in the figure, the largest sum is $1+4+6+7=18$.