#include <bits/stdc++.h>
using namespace std;
const int N=5050;
char s[N][N];
int dist[N][N];
//set<pair<int,int>> can[2*N];
int mv[4][2]={{0,1},{1,0},{-1,0},{0,-1}};
const int M=N*N;
int p[M];
int Find(int x){return x==p[x]?x:p[x]=Find(p[x]);}
void Union(int x,int y){p[Find(x)]=Find(y);}
int idx[N][N];
pair<int,int> cell[M];
int main(){
	int n,k;
	scanf("%i %i",&n,&k);
	for(int i=1;i<=n;i++){
		scanf("%s",s[i]+1);
	}
	int c=0;
	for(int i=1;i<=n;i++){
		for(int j=1;j<=n;j++){
			dist[i][j]=-1;
			idx[i][j]=++c;
			cell[c]={i,j};
			p[c]=c;
		}
	}
	dist[1][1]=0;
	queue<pair<int,int>> q;
	q.push({1,1});
	while(q.size()){
		int x=q.front().first;
		int y=q.front().second;
		if(x==n && y==n)break;
		q.pop();
		for(int i=0;i<4;i++){
			int nx=x+mv[i][0];
			int ny=y+mv[i][1];
			if(s[nx][ny]!='.')continue;
			if(dist[nx][ny]==-1){
				dist[nx][ny]=dist[x][y]+1;
				q.push({nx,ny});
				if(idx[nx+1][ny+1]){
					Union(idx[nx][ny],idx[nx+1][ny+1]);
				}
				//can[nx-ny+N].erase({nx,ny});
			}
		}
		int now=idx[x][y];
		while(true){
			now=Find(now);
			int nx=cell[now].first;
			int ny=cell[now].second;
			if(x+y+k<nx+ny)break;
			if(dist[nx][ny]==-1 && s[nx][ny]=='.'){
				dist[nx][ny]=dist[x][y]+1;
				q.push({nx,ny});
			}
			if(idx[nx+1][ny+1]){
				Union(idx[nx][ny],idx[nx+1][ny+1]);
			}else{
				break;
			}
		}
		now=idx[y+1][x];
		while(true){
			if(now==0)break;
			now=Find(now);
			int nx=cell[now].first;
			int ny=cell[now].second;
			if(x+y+k<nx+ny)break;
			if(dist[nx][ny]==-1 && s[nx][ny]=='.'){
				dist[nx][ny]=dist[x][y]+1;
				q.push({nx,ny});
			}
			if(idx[nx+1][ny+1]){
				Union(idx[nx][ny],idx[nx+1][ny+1]);
			}else{
				break;
			}
		}
	}
	printf("%i\n",dist[n][n]);
	return 0;
}