S[3002][3002],H,W,x,y,z;
main(){
	scanf("%d%d ",&H,&W);
	for(y=1;y<=H;++y)for(x=1;x<=W+1;++x){
		S[y][x]=getchar()=='#'?1e9:0;
	}
	for(y=1;y<=H;++y)for(x=1;x<=W;++x){
		z=fmax(z,S[y][x]=fmin(S[y][x],fmin(S[y-1][x-1],fmin(S[y-1][x],S[y][x-1]))+1));
	}
	printf("%d",-~z/2);
}