#include <iostream>

#include <cstdio>

#include <cstring>

#include <cmath>

#include <algorithm>

#include <queue>

using namespace std;

#define N 10007

struct node

{

    int state,step;

    node(int _state,int _step)

    {

        state = _state;

        step = _step;

    }

    node(){}

};

int vis[<<];

int cut[]; //两个方向

queue<node> que;

int n,m;

int bfs(int s)

{

    int i,j,k;

    memset(vis,,sizeof(vis));

    while(!que.empty())

        que.pop();

    int E = (<<m)-;

    que.push(node(s,));

    vis[s] = ;

    while(!que.empty())

    {

        node tmp = que.front();

        que.pop();

        int state = tmp.state;

        int step = tmp.step;

        int tms = state;

        for(i=;i<;i++) //direction

        {

            for(j=;j<n;j++)  //move

            {

                int end = ((cut[i]>>j) | tms) & E;  // &E : keep m bit

                if(vis[end])

                    continue;

                vis[end] = ;

                if(end == E)

                    return step+;

                que.push(node(end,step+));

            }

            for(j=;j<m;j++)

            {

                int to = ((cut[i]<<j) | tms) & E;

                if(vis[to])

                    continue;

                vis[to] = ;

                if(to == E)

                    return step+;

                que.push(node(to,step+));

            }

        }

    }

    return -;

}

int main()

{

    int i,j;

    char ss[];

    while(scanf("%d",&n)!=EOF)

    {

        cut[] = cut[] = ;

        scanf("%s",ss);

        for(i=;i<=n;i++)

        {

            if(ss[i] == '*')

            {

                cut[] |= (<<i);

                cut[] |= (<<(n--i));

            }

        }

        scanf("%d",&m);

        if(cut[] == )

        {

            puts("-1");

            continue;

        }

        printf("%d\n",bfs());

    }

    return ;

}