#include <cstdio>
#include <cctype>
#include <cstring>
#include <algorithm>
#include <numeric>
#include <vector>
#include <cassert>
#include <array>
using namespace std;

using ll = long long;
using ull = unsigned long long;

#define LOG(f...) fprintf(stderr, f)
#define DBG(f...) printf("%3d: ", __LINE__), printf(f)
// #define DBG(f...) void()
#define all(cont) begin(cont), end(cont)
#ifdef __linux__
#define getchar getchar_unlocked
#define putchar putchar_unlocked
#endif

template <class T> void read(T &x) {
  char ch; x = 0;
  int f = 1;
  while (isspace(ch = getchar()));
  if (ch == '-') ch = getchar(), f = -1;
  do x = x * 10 + (ch - '0'); while (isdigit(ch = getchar()));
  x *= f;
}
template <class T, class ...A> void read(T &x, A&... args) { read(x); read(args...); }

const int N = 100005;

char s[N];
const char *key = "spbsu";
array<int, 26> nxt[N], prv[N];

int main() {
#ifdef LOCAL
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif
  int n;
  read(n);
  scanf("%s", s);
  fill(all(nxt[n]), -1);
  for (int i = n; i; --i) {
    nxt[i - 1] = nxt[i];
    if (i != n) nxt[i - 1][s[i] - 'a'] = i;
  }
  fill(all(prv[0]), -1);
  for (int i = 0; i < n; ++i) {
    prv[i + 1] = prv[i];
    prv[i + 1][s[i] - 'a'] = i;
  }
  int ans = 0x7f7f7f7f;
  for (int i = 2; i < n - 2; ++i) {
    if (s[i] != 'b') continue;
    int pos[5];
    pos[1] = prv[i][key[1] - 'a'];
    pos[0] = pos[1] == -1 ? -1 : prv[pos[1]][key[0] - 'a'];
    pos[3] = nxt[i][key[3] - 'a'];
    pos[4] = pos[3] == -1 ? -1 : nxt[pos[3]][key[4] - 'a'];
    if (pos[0] != -1 && pos[4] != -1)
      ans = min(ans, pos[3] + pos[4] - pos[0] - pos[1] - 6);
  }
  printf("%d\n", ans);
  return 0;
}