#include <bits/stdc++.h>
#define x first
#define y second
#define int long long
#define double long double

using namespace std;
typedef long long LL;
typedef pair<int, int> pii;
typedef pair<pii, int> ppi;
typedef pair<int, pair<int, int>> pip;
typedef const double* (*p_fun)(const double*, int);
const LL N = 2e5 + 10, M = 2e3 + 10, mod = 1e9 + 7, LLF = 1e15,
null = 0x3f3f3f3f3f3f3f;

template <typename T>
bool chmax(T& a, const T& b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
template <typename T, typename... Args>
bool chmax(T& a, const T& b, const Args &...args) {
    bool updated = chmax(a, b);
    return chmax(a, args...) || updated;
}
template <typename T>
bool chmin(T& a, const T& b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template <typename T, typename... Args>
bool chmin(T& a, const T& b, const Args &...args) {
    bool updated = chmin(a, b);
    return chmin(a, args...) || updated;
}
class UnionFind {
public:
    vector<int> parent;
    vector<int> size;
    int n;
    // 当前连通分量数目
    int setCount;

public:
    UnionFind(int _n) : n(_n), setCount(_n), parent(_n), size(_n, 1) {
        iota(parent.begin(), parent.end(), 0);
    }

    int find(int x) { return parent[x] == x ? x : parent[x] = find(parent[x]); }

    bool merge(int x, int y) {
        x = find(x);
        y = find(y);
        if (x == y) return false;
        if (size[x] < size[y]) swap(x, y);
        parent[y] = x;
        size[x] += size[y];
        --setCount;
        return true;
    }

    bool connected(int x, int y) {
        x = find(x);
        y = find(y);
        return x == y;
    }
};
int qmi(int a, int b) {
    int res = 1;
    a %= mod;
    while (b) {
        if (b & 1) res = res * a % mod;
        a = a * a % mod;
        b >>= 1;
    }

    return res;
}
int sqrt(int x) {
    int l = 0, r = 3e9;  // LLONG_MAX
    while (l < r) {
        int mid = (l + r + 1) >> 1;
        if (mid * mid > x)
            r = mid - 1;  // 向下取整
        else
            l = mid;
    }
    return r;
}

bool isrun(int x) {
    if (x % 400 == 0 || (x % 4 == 0 && (x % 100 != 0))) return 1;
    return 0;
}

int yue[] = { 0,31,28,31,30,31,30,31,31,30,31,30,31 };

static inline void solve() {
    int n;
    cin >> n;
    string s;
    cin >> s;
    s = ' ' + s;

    vector<vector<int>> f(n + 1, vector<int>(6)), f2(n + 1, vector<int>(6));
    vector<int> now(6);
    fill(now.begin(), now.end(), -LLF);
    map<char, int> mp;
    string ss = " spbsu";
    mp['s'] = 1, mp['p'] = 2, mp['b'] = 3, mp['u'] = 4;

    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= 4; j++) {
            f[i][j] = now[j];
        }
        now[mp[s[i]]] = i;
    }

    fill(now.begin(), now.end(), LLF);
    for (int i = n; i >= 1; i--) {
        for (int j = 1; j <= 4; j++) {
            f2[i][j] = now[j];
        }
        now[mp[s[i]]] = i;
    }

    int all = LLF;
    for (int i = 1; i <= n; i++) {
        if (s[i] == 'b') {
            int ans = 0;
            if (f[i][2] == -LLF) continue;
            ans += i - f[i][2] - 1;
            int pos = f[i][2];
            if (f[pos][1] == -LLF) continue;
            ans += i - f[pos][1] - 2;

            if (f2[i][1] == LLF) continue;
            ans += f2[i][1] - i - 1;
            pos = f2[i][1];
            if (f2[pos][4] == LLF) continue;
            ans += f2[pos][4] - i - 2;
            chmin(all, ans);
        }
    }

    cout << all;
}
signed main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    cout << unitbuf;
    int t = 1;
    //cin >> t;
    while (t--) {
        solve();
    }
}