#include <bits/stdc++.h>
#define ll int
#define Ma 20210926
#define N 5005
#define mod 998244353
#define PLL pair<ll,ll>
#define x first
#define y second
#define pb push_back
using namespace std;
ll dp[N][N],vis[N][N];
string s[N];
ll n,k;
ll id[N][N];
ll len[N][N];
ll tot=0;
set <ll> st[N];
vector <PLL> v[N];
ll px[4]={1,-1,0,0},py[4]={0,0,1,-1};

void sol()
{
    cin>>n>>k;
    for (ll l=0;l<=n*2-2;l++)
    {
        for (ll i=0;i<n;i++)
        {
            ll j=l-i;
            if (j<0||j>=n)
                continue;
            if (id[i][j]==0)
            {
                id[i][j]=++tot;
                len[i][j]=0;
                st[tot].insert(len[i][j]);
                v[tot].pb({i,j});
            }
            if (j+1<n)
            {
                id[j+1][i]=id[i][j];
                len[j+1][i]=len[i][j]+1;
                st[id[j+1][i]].insert(len[j+1][i]);
                v[id[j+1][i]].pb({j+1,i});
            }
        }

    }
    for (ll i=0;i<n;i++)
        cin>>s[i];
    for (ll i=0;i<n;i++)
        for (ll j=0;j<n;j++)
            if (s[i][j]=='*')
                st[id[i][j]].erase(len[i][j]);
    queue <PLL> q;
    q.push({0,0});
    st[id[0][0]].erase(0);
    vis[0][0]=1;
    while (!q.empty())
    {
        PLL p=q.front();
        //printf("x=%d y=%d dp=%d\n",p.x,p.y,dp[p.x][p.y]);
        q.pop();
        for (ll i=0;i<4;i++){
            ll nx=p.x+px[i],ny=p.y+py[i];
            if (nx<0||nx>=n||ny<0||ny>=n||vis[nx][ny]||s[nx][ny]=='*')
                continue;
            vis[nx][ny]=1,dp[nx][ny]=dp[p.x][p.y]+1;
            q.push({nx,ny});
            st[id[nx][ny]].erase(len[nx][ny]);
        }
        vector <ll> er;
        auto l=st[id[p.x][p.y]].lower_bound(len[p.x][p.y]);
        //printf("l=%d\n",(*l));
        while (l!=st[id[p.x][p.y]].end()&&(*l)<=len[p.x][p.y]+k)
            er.pb((*l)),l++;
        for (auto z:er)
        {
            PLL ok=v[id[p.x][p.y]][z];
            dp[ok.x][ok.y]=dp[p.x][p.y]+1;
            vis[ok.x][ok.y]=1;
            st[id[p.x][p.y]].erase(z);
            q.push(ok);
        }
    }
    if (!vis[n-1][n-1])
    {
        printf("-1\n");
        return;
    }
    printf("%d\n",dp[n-1][n-1]);
}

int main()
{
    ios::sync_with_stdio(0); cin.tie(0);
    ll tt=1;
    //cin>>tt;
    while (tt--)
        sol();
    return 0;
}