1216F · Wi-Fi

Problem: You work as a system administrator in a dormitory, which has $n$ rooms one after another along a straight hallway. Rooms are numbered from $1$ to $n$.

You have to connect all $n$ rooms to the Internet.

You can connect each room to the Internet directly, the cost of such connection for the $i$-th room is $i$ coins.

Some rooms also have a spot for a router. The cost of placing a router in the $i$-th room is also $i$ coins. You cannot place a router in a room which does not have a spot for it. When you place a router in the room $i$, you connect all rooms with the numbers from $max(1,~i - k)$ to $min(n,~i + k)$ inclusive to the Internet, where $k$ is the range of router. The value of $k$ is the same for all routers.

Calculate the minimum total cost of connecting all $n$ rooms to the Internet. You can assume that the number of rooms which have a spot for a router is not greater than the number of routers you have.

Input Format: The first line of the input contains two integers $n$ and $k$ ($1 \le n, k \le 2 \cdot 10^5$) — the number of rooms and the range of each router.

The second line of the input contains one string $s$ of length $n$, consisting only of zeros and ones. If the $i$-th character of the string equals to '1' then there is a spot for a router in the $i$-th room. If the $i$-th character of the string equals to '0' then you cannot place a router in the $i$-th room.

Output Format: Print one integer — the minimum total cost of connecting all $n$ rooms to the Internet.

Note: In the first example it is enough to place the router in the room $3$, then all rooms will be connected to the Internet. The total cost of connection is $3$.

In the second example you can place routers nowhere, so you need to connect all rooms directly. Thus, the total cost of connection of all rooms is $1 + 2 + 3 + 4 + 5 + 6 = 21$.

In the third example you need to connect the room $1$ directly and place the router in the room $3$. Thus, the total cost of connection of all rooms is $1 + 3 = 4$.

In the fourth example you need to place routers in rooms $5$ and $10$. Then all rooms will be connected to the Internet. The total cost of connection is $5 + 10 = 15$.