### Street Watch

Source: http://www.math.utah.edu/~cherk/puzzles.html Problem: Salt Lake City looks like a rectangle crossed with M streets going from North to South and with N streets going from East to West. The city is frequently visited by tourists who suppose to run around in the buses. The Utah governor wants to vigil all moves of the buses. He plans to put policemen at some intersections to watch all the buses moving on the streets visible from that intersections. What is the minimum number of policemen needed for the bus watch? Update (26/03/10) Solution: Posted by Ashu and Aman in comments!!