14. In the triangular prism $A B C - A _ { 1 } B _ { 1 } C _ { 1 }$, $\angle B A C = 90 ^ { \circ }$. Both the front view and side view are squares with side length 1, and the top view is an isosceles right triangle with legs of length 1. Let $M , N , P$ be the midpoints of edges $A B , B C , B _ { 1 } C _ { 1 }$ respectively. Then the volume of the triangular pyramid $P - A _ { 1 } M N$ is \_\_\_\_.