Nguồn:http:/codeforces.com
After the lessons n groups of
schoolchildren went outside and decided to visit Polycarpus to celebrate his
birthday. We know that the i-th group
consists of si friends (1 ≤ si ≤ 4), and they
want to go to Polycarpus together. They decided to get there by taxi. Each car
can carry at most four passengers. What minimum number of cars will the
children need if all members of each group should ride in the same taxi (but
one taxi can take more than one group)?
Input
The first line contains integer n (1 ≤ n ≤ 105) — the number
of groups of schoolchildren. The second line contains a sequence of integers s1, s2, ..., sn (1 ≤ si ≤ 4). The integers
are separated by a space, si is the
number of children in the i-th group.
Output
Print the single number — the minimum number of taxis necessary
to drive all children to Polycarpus.
Examples
input
5
1 2 4 3 3
1 2 4 3 3
output
4
input
8
2 3 4 4 2 1 3 1
2 3 4 4 2 1 3 1
output
5
Note
In the first test we can sort the children into four cars like
this:
·
the third group (consisting of four children),
·
the fourth group (consisting of three children),
·
the fifth group (consisting of three children),
·
the first and the second group (consisting of one and two
children, correspondingly).
There are other ways to sort the groups into four cars.
Tóm tắt
đề:
_ Có n
nhóm học sinh. Nhóm thứ i chứa s[i] học sinh
_ Một
chiếc taxi chỉ có tối đa 4 chỗ ngồi.
_ Các học
sinh trong cùng 1 nhóm thì lại muốn ngồi chung trong 1 chiếc taxi. Và 1 chiếc
taxi có thể chứa nhiều hơn 1 nhóm học sinh.
_ Hỏi :
Cần ít nhất bao nhiêu chiếc taxi để chở hết toàn bộ n nhóm học sinh này.
Solution by ĐNK
