package adv2017;
import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.InputMismatchException;

public class N608_2 {
	InputStream is;
	PrintWriter out;
	String INPUT = "";
	
	void solve()
	{
		int x = ni()-1;
		char[] s = ns().toCharArray();
		long ret = Math.min(go1(s, x), go2(s, x));
		if(ret == Long.MAX_VALUE)ret = 3;
		out.println(ret);
	}

	long go1(char[] s, int x)
	{
		int n = s.length;
		int p = 0, q = n-1;
		int[] f = new int[n];
		while(p < n && (s[p] == '#') == (f[p] == 1))p++;
		if(p >= x)return Long.MAX_VALUE;
		long ret = x - p;
		for(int i = p;i < x;i++){
			f[i] ^= 1;
		}
		while(q >= 0 && (s[q] == '#') == (f[q] == 1))q--;
		if(p > q)return ret;
		for(int i = p+1;i <= q;i++){
			f[i] ^= 1;
			ret++;
			if((f[i] == 1) == (s[i] == '#')){
				continue;
			}
			if(i+1 == q){
				ret += 2;
				break;
			}
			f[i] ^= 1;
			f[i+1] ^= 1;
			ret += 2;
		}
		return ret;
	}
	

	long go2(char[] s, int x)
	{
		int n = s.length;
		int p = 0, q = n-1;
		int[] f = new int[n];
		while(q >= 0 && (s[q] == '#') == (f[q] == 1))q--;
		if(q <= x)return Long.MAX_VALUE;
		long ret = q - x;
		for(int i = x+1;i <= q;i++){
			f[i] ^= 1;
		}
		while(p < n && (s[p] == '#') == (f[p] == 1))p++;
		if(p > q)return ret;
		for(int i = q-1;i >= p;i--){
			f[i] ^= 1;
			ret++;
			if((f[i] == 1) == (s[i] == '#')){
				continue;
			}
			if(i-1 == p){
				ret += 2;
				break;
			}
			ret += 2;
			f[i] ^= 1;
			f[i-1] ^= 1;
		}
		return ret;
	}

public static int sumFenwick(int[] ft, int i)
{
	int sum = 0;
	for(i++;i > 0;i -= i&-i)sum ^= ft[i];
	return sum;
}

public static void addFenwick(int[] ft, int i, int v)
{
	if(v == 0 || i < 0)return;
	int n = ft.length;
	for(i++;i < n;i += i&-i)ft[i] ^= v;
}

public static int[] restoreFenwick(int[] ft)
{
	int n = ft.length-1;
	int[] ret = new int[n];
	for(int i = 0;i < n;i++)ret[i] = sumFenwick(ft, i);
	for(int i = n-1;i >= 1;i--)ret[i] ^= ret[i-1];
	return ret;
}


	
	void run() throws Exception
	{
		is = INPUT.isEmpty() ? System.in : new ByteArrayInputStream(INPUT.getBytes());
		out = new PrintWriter(System.out);
		
		long s = System.currentTimeMillis();
		solve();
		out.flush();
		if(!INPUT.isEmpty())tr(System.currentTimeMillis()-s+"ms");
//		Thread t = new Thread(null, null, "~", Runtime.getRuntime().maxMemory()){
//			@Override
//			public void run() {
//				long s = System.currentTimeMillis();
//				solve();
//				out.flush();
//				if(!INPUT.isEmpty())tr(System.currentTimeMillis()-s+"ms");
//			}
//		};
//		t.start();
//		t.join();
	}
	
	public static void main(String[] args) throws Exception { new N608_2().run(); }
	
	private byte[] inbuf = new byte[1024];
	public int lenbuf = 0, ptrbuf = 0;
	
	private int readByte()
	{
		if(lenbuf == -1)throw new InputMismatchException();
		if(ptrbuf >= lenbuf){
			ptrbuf = 0;
			try { lenbuf = is.read(inbuf); } catch (IOException e) { throw new InputMismatchException(); }
			if(lenbuf <= 0)return -1;
		}
		return inbuf[ptrbuf++];
	}
	
	private boolean isSpaceChar(int c) { return !(c >= 33 && c <= 126); }
	private int skip() { int b; while((b = readByte()) != -1 && isSpaceChar(b)); return b; }
	
	private double nd() { return Double.parseDouble(ns()); }
	private char nc() { return (char)skip(); }
	
	private String ns()
	{
		int b = skip();
		StringBuilder sb = new StringBuilder();
		while(!(isSpaceChar(b))){ // when nextLine, (isSpaceChar(b) && b != ' ')
			sb.appendCodePoint(b);
			b = readByte();
		}
		return sb.toString();
	}
	
	private char[] ns(int n)
	{
		char[] buf = new char[n];
		int b = skip(), p = 0;
		while(p < n && !(isSpaceChar(b))){
			buf[p++] = (char)b;
			b = readByte();
		}
		return n == p ? buf : Arrays.copyOf(buf, p);
	}
	
	private int[] na(int n)
	{
		int[] a = new int[n];
		for(int i = 0;i < n;i++)a[i] = ni();
		return a;
	}
	
	private long[] nal(int n)
	{
		long[] a = new long[n];
		for(int i = 0;i < n;i++)a[i] = nl();
		return a;
	}
	
	private char[][] nm(int n, int m) {
		char[][] map = new char[n][];
		for(int i = 0;i < n;i++)map[i] = ns(m);
		return map;
	}
	
	private int[][] nmi(int n, int m) {
		int[][] map = new int[n][];
		for(int i = 0;i < n;i++)map[i] = na(m);
		return map;
	}
	
	private int ni() { return (int)nl(); }
	
	private long nl()
	{
		long num = 0;
		int b;
		boolean minus = false;
		while((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'));
		if(b == '-'){
			minus = true;
			b = readByte();
		}
		
		while(true){
			if(b >= '0' && b <= '9'){
				num = num * 10 + (b - '0');
			}else{
				return minus ? -num : num;
			}
			b = readByte();
		}
	}
	
	private static void tr(Object... o) { System.out.println(Arrays.deepToString(o)); }
}