def seq ():
    var n, x1, x2, r

    x1 = n
    while x1 > 0:
        r = unif(-2,1)
        x1 = x1 + r
        tick 1

        x2 = n
        while x2 > 0:
            r = unif(-2,1)
            x2 = x2 + r
            tick 1