Scientific Project

The scientific project consists of $N$ tasks, each executed on one of $N$ processors of a supercomputer. Tasks may run concurrently, but some can only start after certain other tasks are completed. For each task, its execution time and the list of prerequisite tasks (which may be empty) are given.

Find the minimal possible time required to complete all $N$ tasks. Output -1 if it is impossible to complete the project. All input numbers are positive integers not exceeding $100$.