// 2009アジア地区予選 J : Infected Land
#include <iostream>
#include <fstream>
#include <vector>
#include <string>
#include <set>
#include <queue>

using namespace std;

char dx[] = {-1,-1,-1,0,1,1,1,0};
char dy[] = {-1,0,1,1,1,0,-1,-1};

class Node{
public:
	int land, x, y;
	Node(int land, int x, int y) : land(land), x(x), y(y) {}
	bool operator < (const Node &n) const { 
		return make_pair(land,make_pair(x,y)) < make_pair(n.land,make_pair(n.x,n.y));
	}
};

int main(){
	ifstream fin("J.in");
	int n;
	string s[5];
	while(fin >> n, n){
		for(int i=0;i<n;i++) fin >> s[i];
		int sx, sy;
		for(int i=0;i<n;i++)
			for(int j=0;j<n;j++)
				if(s[i][j]=='@') sx = i, sy = j;
		int land = 0;
		for(int i=n-1;i>=0;i--)
			for(int j=n-1;j>=0;j--)
				land = 2*land + (s[i][j]=='#');
		int ans = -1;
		set<Node> mem; mem.insert(Node(land, sx, sy));
		queue< pair<Node,int> > qu; qu.push(make_pair(Node(land,sx,sy),0));
		while(!qu.empty()){
			pair<Node,int> p = qu.front(); qu.pop();
			int L = p.first.land, x = p.first.x, y = p.first.y, cost = p.second;
			if(L==0){ ans = cost; break; }
			for(int dir=0;dir<8;dir++){
				int nx = x+dx[dir], ny = y+dy[dir];
				if(nx<0||n<=nx||ny<0||n<=ny||(L>>(nx*n+ny))&1) continue;
				int nL = 0;
				for(int i=n-1;i>=0;i--){
					for(int j=n-1;j>=0;j--){
						if(i==nx&&j==ny) continue;
						int cnt = 0;
						for(int k=0;k<8;k++){
							int ni = i+dx[k], nj = j+dy[k];
							if(ni<0||n<=ni||nj<0||n<=nj) continue;
							if(ni==nx&&nj==ny) cnt++;
							else cnt += (L>>(ni*n+nj))&1;
						}
						if((L>>(i*n+j))&1){
							if(cnt==2||cnt==3) nL |= 1<<(i*n+j);
						} else {
							if(cnt==3) nL |= 1<<(i*n+j);
						}
					}
				}
				if(mem.find(Node(nL,nx,ny))==mem.end()){
					mem.insert(Node(nL,nx,ny));
					qu.push(make_pair(Node(nL,nx,ny),cost+1));
				}
			}
		}
		cout << ans << endl;
	}
}