You want to schedule a list of jobs in d days. Jobs are dependent 
(i.e to work on the i-th job, you have to finish all the jobs j where 0 <= j < i). 
You have to finish at least one task every day. The difficulty of a job schedule 
is the sum of difficulties of each day of the d days. 
The difficulty of a day is the maximum difficulty of a job done in that day.
Given an array of integers jobDifficulty and an integer d. 
The difficulty of the i-th job is jobDifficulty[i].

Return the minimum difficulty of a job schedule. If you cannot find a schedule for the jobs return -1.

Example: 

Input: jobDifficulty = [6,5,4,3,2,1], d = 2
Output: 7
Explanation: First day you can finish the first 5 jobs, total difficulty = 6.
Second day you can finish the last job, total difficulty = 1.
The difficulty of the schedule = 6 + 1 = 7