Given a binary string of length , where exactly
characters are ones,
compute the number of substrings that have at least three ones.
Constraints
are presented in sorted order.
In tests worth marks, you may assume
.
In tests worth an additional marks, you may assume
.
Input Specification
The first line of the input contains two space-separated integers, and
.
Each of the next lines contains a single integer,
, indicating that character
of the string is a one.
Output Specification
Output a single integer, the number of substrings that contain at least three ones.
Sample Input
5 4
1
2
4
5
Sample Output
3
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