40'

#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
const ll _N = 200000 + 1;
ll N, A, t = 0, s = 0, _l = 0, _r, l[_N], r[_N];
bool d = 0;
string S;
signed main(){
	ios::sync_with_stdio(0), cin.tie(), cout.tie();
	cin >> N >> A >> S;
	_r = N + 1, S = 'x' + S + 'x';
	l[0] = 0, r[N + 1] = N + 1;
	for (ll i = 1; i <= N; i++){
		l[i] = _l;
		if (S[i] == '#') s++, _l = i;
	}l[N + 1] = _l;
	for (ll i = N; i >= 1; i--){
		r[i] = _r;
		if (S[i] == '#') _r = i;
	}r[0] = _r;
	while (1){
		if (d) t += A - l[A], A = l[A]; else t += r[A] - A, A = r[A];
		if (S[A] == 'x') d = !d;
		else if (S[A] == '#'){
			r[l[A]] = r[A], l[r[A]] = l[A];
			S[A] = '.', d = !d, s--;
		}if (!s) break;
	}cout << t;
	return 0;
}