#include <bits/stdc++.h>

#define show(x) cerr << #x << " = " << x << endl

using namespace std;
using ll = long long;
using pii = pair<int, int>;
using vi = vector<int>;


template <typename T>
ostream& operator<<(ostream& os, const vector<T>& v)
{
    os << "sz=" << v.size() << "\n[";
    for (const auto& p : v) {
        os << p << ",";
    }
    os << "]\n";
    return os;
}

template <typename T>
ostream& operator<<(ostream& os, const set<T>& v)
{
    os << "sz=" << v.size() << "\n[";
    for (const auto& p : v) {
        os << p << ",";
    }
    os << "]\n";
    return os;
}

template <typename S, typename T>
ostream& operator<<(ostream& os, const pair<S, T>& p)
{
    os << "(" << p.first << "," << p.second
       << ")";
    return os;
}


constexpr ll MOD = 1e9 + 7;

template <typename T>
constexpr T INF = numeric_limits<T>::max() / 100;

int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);

    int N;
    cin >> N;
    N--;
    string S;
    cin >> S;
    const int size = S.size();


    auto solve1 = [&]() {
        if (N > 1000) {
            return INF<int>;
        }
        vector<bool> need(size);
        set<int> st;
        for (int i = 0; i < size; i++) {
            need[i] = S[i] == '#';
            if (need[i]) {
                st.insert(i);
            }
        }
        int pos = N;
        bool l = true;
        int ans = 0;
        while (not st.empty()) {
            const int left = *st.cbegin();
            const int right = *st.crbegin();
            if (l) {
                for (; pos > left; pos--) {
                    ans++;
                    if (need[pos - 1]) {
                        st.erase(pos - 1);
                    } else {
                        st.insert(pos - 1);
                    }
                    need[pos - 1] = not need[pos - 1];
                }
            } else {
                for (; pos < right; pos++) {
                    ans++;
                    if (need[pos + 1]) {
                        st.erase(pos + 1);
                    } else {
                        st.insert(pos + 1);
                    }
                    need[pos + 1] = not need[pos + 1];
                }
            }
            l = not l;
        }
        return ans;
    };

    auto solve2 = [&]() {
        if (N > 1000) {
            return INF<int>;
        }
        vector<bool> need(size);
        set<int> st;
        for (int i = 0; i < size; i++) {
            need[i] = S[i] == '#';
            if (need[i]) {
                st.insert(i);
            }
        }
        int pos = N;
        bool l = false;
        int ans = 0;
        while (not st.empty()) {
            const int left = *st.cbegin();
            const int right = *st.crbegin();
            if (l) {
                for (; pos > left; pos--) {
                    ans++;
                    if (need[pos - 1]) {
                        st.erase(pos - 1);
                    } else {
                        st.insert(pos - 1);
                    }
                    need[pos - 1] = not need[pos - 1];
                }
            } else {
                for (; pos < right; pos++) {
                    ans++;
                    if (need[pos + 1]) {
                        st.erase(pos + 1);
                    } else {
                        st.insert(pos + 1);
                    }
                    need[pos + 1] = not need[pos + 1];
                }
            }
            l = not l;
        }
        return ans;
    };

    auto solve3 = [&]() {
        vector<bool> need(size);
        set<int> st;
        for (int i = 0; i < size; i++) {
            need[i] = S[i] == '#';
            if (need[i]) {
                st.insert(i);
            }
        }
        int pos = N;
        bool l = true;
        int ans = 0;
        int cnt = 0;
        while (not st.empty() and cnt < 2) {
            const int left = *st.cbegin();
            const int right = *st.crbegin();
            if (l) {
                for (; pos > left; pos--) {
                    ans++;
                    if (need[pos - 1]) {
                        st.erase(pos - 1);
                    } else {
                        st.insert(pos - 1);
                    }
                    need[pos - 1] = not need[pos - 1];
                }
            } else {
                for (; pos < right; pos++) {
                    ans++;
                    if (need[pos + 1]) {
                        st.erase(pos + 1);
                    } else {
                        st.insert(pos + 1);
                    }
                    need[pos + 1] = not need[pos + 1];
                    if (need[pos]) {
                        ans += (pos == right - 1 ? 1 : 2);
                        need[pos] = not need[pos];
                        st.erase(pos);
                        need[pos + 1] = (pos == right - 1 ? need[pos + 1] : not need[pos + 1]);
                        if (need[pos + 1]) {
                            st.insert(pos + 1);
                        }
                    }
                }
            }
            l = not l;
            cnt++;
        }
        return ans;
    };

    auto solve4 = [&]() {
        vector<bool> need(size);
        set<int> st;
        for (int i = 0; i < size; i++) {
            need[i] = S[i] == '#';
            if (need[i]) {
                st.insert(i);
            }
        }
        int pos = N;
        bool l = false;
        int ans = 0;
        int cnt = 0;
        while (not st.empty() and cnt < 2) {
            const int left = *st.cbegin();
            const int right = *st.crbegin();
            if (l) {
                for (; pos > left; pos--) {
                    ans++;
                    if (need[pos - 1]) {
                        st.erase(pos - 1);
                    } else {
                        st.insert(pos - 1);
                    }
                    need[pos - 1] = not need[pos - 1];
                    if (need[pos]) {
                        ans += (pos == left + 1 ? 1 : 2);
                        need[pos] = not need[pos];
                        st.erase(pos);
                        need[pos - 1] = (pos == left + 1 ? need[pos - 1] : not need[pos - 1]);
                        if (pos > left + 1) {
                            st.insert(pos - 1);
                        }
                    }
                }
            } else {
                for (; pos < right; pos++) {
                    ans++;
                    if (need[pos + 1]) {
                        st.erase(pos + 1);
                    } else {
                        st.insert(pos + 1);
                    }
                    need[pos + 1] = not need[pos + 1];
                }
            }
            l = not l;
            cnt++;
        }
        return ans;
    };

    cout << min({solve1(), solve2(), solve3(), solve4()}) << endl;
    return 0;

    //    cout << ans << endl;

    return 0;
}