function [p1,p2] = position(p1,p2)

    d = rand;
    if d<=0.25 
        p1 = p1-1;
    elseif d>0.25 & d <=0.5
        p1 = p1 + 1;
    elseif d>0.5 & d <=0.75
        p2 = p2-1;
    else
        p2 = p2+1;
    end