Every morning, Shekhar leaves his house and walks in the direction of the sun in such a way that its shadow forms behind him. He walks 12 km to reach his office. In the evening just before sunset, he leaves his office and then walks in the direction of the sun in such a way that his shadow is formed exactly behind him. He walks 9 km and then takes a left turn and walks 4 km more to reach his gym. What is the shortest distance between his gym and his house?