#define max(p,q)(p>q?p:q)
#define min(p,q,r)(p<q?p<r?p:r:q<r?q:r)
a[3010][3010];
j,t,s;
main(i){
	gets(&t);
	while(t=~getchar()){
		j++;
		if(t==~'#'){
			a[i][j]=min(a[i-1][j],a[i-1][j-1],a[i][j-1])+1;
			s=max(s,a[i][j]);
		}
		if(t==~'\n'){i++;j=0;}
	}
	printf("%d",-~s/2);
	return 0;
}