When we construct a pregram by using a computer, we sometimes interrupt to work and store the program constructed up to this point on a disk or another medium, At that time, we sometimes make abackup copy ofthe program on another disk. Whether we makeabackup copy or not seems to depend on the probability that the program on a disk wM be broken by an accident during the work. This kind of problem is formulated by dynaJnic prograJnming and the optima] strategy is derived with the critical values whose explicit form is analytically derived.