/* -*- coding: utf-8 -*-
 *
 * 2073.cc:  No.2073 Concon Substrings (Swap Version) - yukicoder
 */

#include<cstdio>
#include<algorithm>
 
using namespace std;

/* constant */

const int MAX_N = 200000;
const int MAX_N3 = MAX_N * 3;

enum { C = 2, O = 14, N = 13 };

/* typedef */

/* global variables */

char s[MAX_N3 + 4];
int cs[3][26];

/* subroutines */

/* main */

int main() {
  int n;
  scanf("%d%s", &n, s);
  int n3 = n * 3;

  for (int i = 0; i < n3; i++) cs[i % 3][s[i] - 'a']++;

  int sum = 0;
  for (int i = 0; i < 3; i++) {
    int i1 = (i + 1) % 3, i2 = (i + 2) % 3;
    sum += min(cs[i][C], min(cs[i1][O], cs[i2][N]));
  }

  printf("%d\n", sum);

  //for (int i = 0; i < 3; i++)
  //printf("%d: %d,%d,%d\n", i, cs[i][C], cs[i][O], cs[i][N]);
  return 0;
}