有一段木板长度为L，和若干小段a，b，c，d…..要对其进行截断，最终得到长度为a，b，c，d。。。的小木板，截断时需要一定的费用，截断长度为n的木板的费用就为n，要求最后德尔总费用最低。-Farmer John wants to repair a small length of the fence around the pasture. He measures the fence and finds that he needs N (1 ≤ N ≤ 20,000) planks of wood, each having some integer length Li (1 ≤ Li ≤ 50,000) units. He then purchases a single long board just long enough to saw into the N planks (i.e., whose length is the sum of the lengths Li). FJ is ignoring the "kerf", the extra length lost to sawdust when a sawcut is made　you should ignore it, too.