时间限制：C/C++/Rust/Pascal 1秒，其他语言2秒
空间限制：C/C++/Rust/Pascal 256 M，其他语言512 M
64bit IO Format: %lld
空间限制：C/C++/Rust/Pascal 256 M，其他语言512 M
64bit IO Format: %lld
题目描述
Bobo has a string 
of length n consisting of only digits 0, 1, and 2, and he wants to pick some *disjoint* subsequences which equal to 2020, as many as possible.
Formally, Bobo would like to find k quadrangle%2C%20%5Cdots%2C%20(a_k%2C%20b_k%2C%20c_k%2C%20d_k))
where
*
*
*
for 
.
Find the maximum value of k.
Formally, Bobo would like to find k quadrangle
*
*
*
Find the maximum value of k.
输入描述:
The input consists of several test cases terminated by end-of-file.
The first line of each test case contains an integer n and the second line contains a string
*
*
* The sum of n does not exceed.
输出描述:
For each test case, print an integer which denotes the result.