using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

public class MainClass
{
	public const long Giri = 1000000007;
	public const long DpInf = 99999999999;
	
	public class Term
	{
		public char Color {get;set;}
		public int Start {get;set;}
		public int End {get;set;}
	}
	
	public static void Main(string[] args)
	{
		var n = Console.ReadLine().ToInt();
		var s = Console.ReadLine();
		//[a,b)
		var terms = new List<Term>();
		char k = s[0];
		int start = 0;
		for(int i = 0; i < n; i++)
		{
			if(k != s[i])
			{
				terms.Add(new Term() {Color = k, Start = start, End = i});
				k = s[i];
				start = i;
			}
		}
		terms.Add(new Term() {Start = start, End = n, Color=s[n-1]});
		
		int black = 0;
		int white = 0;
		for(int i = 0; i < terms.Count; i++)
		{
			if(terms[i].Color == '#')
				black += terms[i].End - terms[i].Start;
			else
				white += terms[i].End - terms[i].Start;
		}
		if(black == 0 || white == 0 || (terms.Count > 1 && terms[0].Color == '.'))
		{
			Console.WriteLine(0);
			return;
		}
		Console.WriteLine(Math.Min(black, white));
		
	}

	static long GCD(long a, long b)
	{
		var v = new[] { a, b };
		while (v[1] != 0) { v = new[] { v[1], v[0] % v[1] }; }
		return v[0];
	}
	public static string[] Input() { return Console.ReadLine().Split(); }
}

public static class Smarter
{
	public static int ToInt(this string str) { return int.Parse(str); }
	public static long ToLong(this string str) { return long.Parse(str); }
	public static int[] ToInt(this string[] str)
	{
		var r = new int[str.Length];
		for(int i = 0; i < str.Length; i++) r[i] = str[i].ToInt();
		return r;
	 }
}
