def foo(set, subset, iter):
    if len(subset) == 0:
        return iter
    if len(set) == 0 or len(set) < len(subset):
        return -1
    if set[0] == subset[0]:
        return foo(set[1:], subset[1:], iter)
    else:
        return foo(set[1:], subset, iter+1)