#include <iostream>
#include <vector>

using namespace std;

int main() {
    long long  n;
    cin >> n;
    vector <long long> a(n + 1, -1001);
    char read;
    cin >> read;
    a[1] = 0;
    for (long long int i = 2; i <= n; ++i) {
        cin >> read;
        if (read == 'w')
            continue;
        if (i == 2)
            a[i] = 0;
        if (i == 3)
            a[i] = max(a[0], a[2]);
        if (i == 4)
            a[i] = max(a[1], a[3]);
        if (i >=5)
            a[i] = max(max(a[i - 1], a[i - 3]), a[i-5]);
        if(read == '"') a[i]++;
    }
    if (a[n] < 0)
        cout << "-1";
    else
        cout << a[n];
    return 0;
}