Drawing on Erastothenes' method, if two observers are due north and south of each other and are separated by 400 km, what is the circumference of

Drawing on Erastothenes' method, if two observers are due north and south of each other and are separated by 400 km, what is the circumference of their spherical world if they see the same star on their meridian at altitudes of 23 degrees and 47 degrees respectively, and at the exact same time?