#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;

typedef int _loop_int;
#define REP(i,n) for(_loop_int i=0;i<(_loop_int)(n);++i)
#define FOR(i,a,b) for(_loop_int i=(_loop_int)(a);i<(_loop_int)(b);++i)
#define FORR(i,a,b) for(_loop_int i=(_loop_int)(b)-1;i>=(_loop_int)(a);--i)

#define DEBUG(x) cout<<#x<<": "<<x<<endl
#define DEBUG_VEC(v) cout<<#v<<":";REP(i,v.size())cout<<" "<<v[i];cout<<endl
#define ALL(a) (a).begin(),(a).end()

#define CHMIN(a,b) a=min((a),(b))
#define CHMAX(a,b) a=max((a),(b))

// mod
const ll MOD = 1000000007ll;
#define FIX(a) ((a)%MOD+MOD)%MOD

// floating
typedef double Real;
const Real EPS = 1e-11;
#define EQ0(x) (abs(x)<EPS)
#define EQ(a,b) (abs(a-b)<EPS)
typedef complex<Real> P;

int n;
int l;
char s[125252];

int p;
char t[125252];
int cnt;
void flip(){
  t[p] = t[p]=='.' ? '#' : '.';
  // puts(t);
}
void mr(){
  p++;
  flip();
  cnt++;
}
void ml(){
  p--;
  flip();
  cnt++;
}

int solve(){
  p = n;
  REP(i,l)t[i] = '.';
  cnt = 0;
  int leftmost = 0;
  while(s[leftmost]==t[leftmost])leftmost++;
  if(leftmost > p)return 1252183025;
  if(leftmost==p && p==l-1)return 1252183025;
  if(leftmost == p){
    mr();
  }
  while(leftmost+1 < p){
    ml();
  }
  int rightmost = l-1;
  while(s[rightmost]==t[rightmost])rightmost--;
  while(p < rightmost){
    if(s[p-1]!=t[p-1]){
      ml();mr();
    }
    mr();
  }
  while(s[p-1]!=t[p-1]||s[p]!=t[p]){
    ml();
    if(s[p]!=t[p]||s[p+1]!=t[p+1])mr();
  }
  return cnt;
}

int solve3(){
  p = n;
  REP(i,l)t[i] = '.';
  cnt = 0;
  int leftmost = 0;
  while(s[leftmost]==t[leftmost])leftmost++;
  if(leftmost > p)return 1252183025;
  if(leftmost==p && p==l-1)return 1252183025;
  if(leftmost == p){
    mr();
  }
  while(leftmost+1 < p){
    ml();
    if(s[p+1]!=t[p+1]){
      mr();ml();
    }
  }
  int rightmost = l-1;
  while(s[rightmost]==t[rightmost])rightmost--;
  while(p < rightmost){
    if(s[p-1]!=t[p-1]){
      ml();mr();
    }
    mr();
  }
  while(s[p-1]!=t[p-1]||s[p]!=t[p]){
    ml();
    if(s[p]!=t[p]||s[p+1]!=t[p+1])mr();
  }
  return cnt;
}

int solve2(){
  int ans = solve();
  CHMIN(ans,solve3());
  reverse(s,s+l);
  n = l-1-n;
  CHMIN(ans,solve());
  CHMIN(ans,solve3());
  return ans;
}

int main(){
  scanf("%d",&n);
  n--;
  scanf("%s",s);
  l = strlen(s);
  printf("%d\n",solve2());
  return 0;
}