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

#ifdef CKISEKI
#define safe cerr<<__PRETTY_FUNCTION__<<" line "<<__LINE__<<" safe\n"
#define debug(a...) debug_(#a, a)
#define orange(a...) orange_(#a, a)
template <typename ...T>
void debug_(const char *s, T ...a) {
    cerr << "\e[1;32m(" << s << ") = (";
    int cnt = sizeof...(T);
    (..., (cerr << a << (--cnt ? ", " : ")\e[0m\n")));
}
template <typename I>
void orange_(const char *s, I L, I R) {
    cerr << "\e[1;32m[ " << s << " ] = [ ";
    for (int f = 0; L != R; ++L)
        cerr << (f++ ? ", " : "") << *L;
    cerr << " ]\e[0m\n";
}
#else
#define safe ((void)0)
#define debug(...) safe
#define orange(...) safe
#endif

const int inf = 1e9;

int main() {
    cin.tie(nullptr) -> sync_with_stdio(false);
    int n;
    string S;
    cin >> n >> S;

    string P = "spbsu";

    S = "$" + S + "$";
    vector<array<int,5>> prv(n + 2);
    array<int,5> last = {-1, -1, -1, -1, -1};
    for (int i = 1; i <= n + 1; i++) {
        for (int j = 0; j < 5; j++)
            if (P[j] == S[i])
                last[j] = i;

        prv[i] = last;
    }
    vector<array<int,5>> nxt(n + 2);
    last = {-1, -1, -1, -1, -1};
    nxt[n+1] = last;
    prv[0] = last;

    for (int i = n; i >= 0; i--) {
        for (int j = 0; j < 5; j++)
            if (P[j] == S[i])
                last[j] = i;
        nxt[i] = last;
    }

    int ans = inf;
    for (int i = 1; i <= n; i++) {
        for (int s = 0; s < 5; s++) {
            int cost = 0;
            for (int j = s, x = i; j >= 0; j--) {
                x = prv[x][j];
                if (x == -1) {
                    cost = inf;
                    goto end;
                }
                cost += i - (s - j) - x;
                if (i == 1 && s == 0)
                    debug(i - (s - j) - x);
            }
            for (int j = s+1, x = i + 1; j < 5; j++) {
                x = nxt[x][j];
                if (x == -1) {
                    cost = inf;
                    goto end;
                }
                cost += x - i - (j - s);
                if (i == 1 && s == 0)
                    debug(x - i - (j - s));
            }
end:

            if (cost == inf) continue;
            debug(i, s, cost);
            ans = min(ans, cost);
        }
    }

    cout << ans << '\n';
}