// coached by rainboy
#include <algorithm>
#include <iostream>
using namespace std;
const int N = 5000;
const int A = 10;
const int INF = 0x3f3f3f3f;
char aa[N + 1];
int qq[N][A], dp[N][N];
int main() {
ios_base::sync_with_stdio(false), cin.tie(NULL);
int n; cin >> n >> aa;
for (int i = n - 1; i >= 0; i--) {
aa[i] -= 'a';
if (i + 1 < n) {
for (int a = 0; a < A; a++)
qq[i][a] = qq[i + 1][a];
qq[i][(int) aa[i + 1]] = i + 1;
} else
for (int a = 0; a < A; a++)
qq[i][a] = n;
}
for (int i = 0; i < n; i++)
for (int j = 0; j < n; j++)
dp[i][j] = INF;
dp[0][0] = 0;
for (int j = 0; j < n; j++)
for (int i = 0; i < n; i++) {
int x = dp[i][j];
if (x == INF)
continue;
for (int a = 0; a < A; a++)
if (a != 4) {
int i_ = qq[i][a];
if (i_ < n)
dp[i_][j] = min(dp[i_][j], x + 2);
}
int i_ = qq[j][4];
if (i_ > i)
continue;
int i__ = i + 1;
for (int a = 0; a < A; a++)
if (a != 4)
i__ = min(i__, qq[i_][a]);
dp[i__][i] = min(dp[i__][i], x + i - i_);
}
int ans = INF;
for (int j = n - 1; j >= 0; j--) {
for (int i = 0; i <= j; i++)
ans = min(ans, dp[i][j]);
if (aa[j] == 4)
break;
}
for (int i = 0; i < n; i++)
if (aa[i] == 4)
ans++;
cout << ans << '\n';
return 0;
}
