Initially, there are 3⁸⁰ particles at the origin (0,0) . At each step the particles are moved to points above the x -axis as follows: if there are n particles at any point ( x, y ), then [ n 3 ] of them are moved to (x+1, y+1), [ n 3 ] are moved to (x, y+1) and the remaining to (x-1, y+1) . For example, after the first step, there are 3 ⁷⁹ particles each at (1,1),(0,1) and (-1,1) . After the second step, there are 3⁷