CF439A Devu, the Singer and Churu, the Joker

Devu is a renowned classical singer. He is invited to many big functions/festivals. Recently he was invited to "All World Classical Singing Festival". Other than Devu, comedian Churu was also invited. Devu has provided organizers a list of the songs and required time for singing them. He will sing $ n $ songs, $ i^{th} $ song will take $ t_{i} $ minutes exactly. The Comedian, Churu will crack jokes. All his jokes are of 5 minutes exactly. People have mainly come to listen Devu. But you know that he needs rest of 10 minutes after each song. On the other hand, Churu being a very active person, doesn't need any rest. You as one of the organizers should make an optimal sсhedule for the event. For some reasons you must follow the conditions: - The duration of the event must be no more than $ d $ minutes; - Devu must complete all his songs; - With satisfying the two previous conditions the number of jokes cracked by Churu should be as many as possible. If it is not possible to find a way to conduct all the songs of the Devu, output -1. Otherwise find out maximum number of jokes that Churu can crack in the grand event.

The first line contains two space separated integers $ n $ , $ d $ $ (1

If there is no way to conduct all the songs of Devu, output -1. Otherwise output the maximum number of jokes that Churu can crack in the grand event.

Consider the first example. The duration of the event is $ 30 $ minutes. There could be maximum $ 5 $ jokes in the following way: - First Churu cracks a joke in $ 5 $ minutes. - Then Devu performs the first song for $ 2 $ minutes. - Then Churu cracks $ 2 $ jokes in $ 10 $ minutes. - Now Devu performs second song for $ 2 $ minutes. - Then Churu cracks $ 2 $ jokes in $ 10 $ minutes. - Now finally Devu will perform his last song in $ 1 $ minutes. Total time spent is $ 5+2+10+2+10+1=30 $ minutes. Consider the second example. There is no way of organizing Devu's all songs. Hence the answer is -1.