def even_odd(v,epsilon):
    i = 1
    x = 0.1
    summ = 0
    while summ<v:
        summ = summ + x
        if i%2==0:
            x = x + 0.1
        else:
            x = x + 0.2
        i = i + 1
    if abs(summ-v)<epsilon:
        return i-1
    else:
        return -1