#include <algorithm>
#include <iostream>
using namespace std;
const int N = 500;
const int A = 10;
const int INF = 0x3f3f3f3f;
char aa[N + 1];
int qq[N][A], dp[N][N], dd[N], qu[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 = i; j < n; j++)
dp[i][j] = INF;
dp[0][0] = 0;
for (int i = 0; i < n; i++) {
int i_ = n;
if (aa[i] != 4)
i_ = i;
else
for (int a = 0; a < A; a++)
if (a != 4)
i_ = min(i_, qq[i][a]);
for (int j = i_; j < n; j++)
dd[j] = INF;
dd[i_] = 0;
for (int j = i_; j < n; j++) {
int d = dd[j] + 1;
for (int a = 0; a < A; a++)
if (a != 4) {
int k = qq[j][a];
if (k < n)
dd[k] = min(dd[k], d);
}
}
for (int j = i; j < n; j++) {
int x = dp[i][j];
if (x == INF)
continue;
int q = qq[j][4];
if (q == n)
continue;
for (int k = max(q, i_); k < n; k++)
if (dd[k] != INF)
dp[q][k] = min(dp[q][k], x + dd[k] * 2 + k - q);
}
}
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;
}
