Rules:

• • You can always place a pebble on position 1
• • You can place a pebble on position i only if position (i-1) has a pebble
• • You can remove a pebble from position i only if position (i-1) has a pebble
• • The pebble on position 1 is always removable
• • Goal: Place a pebble on the largest possible position