//#include <cstdint>
#include <cstdlib>
#include <algorithm>
#include <iostream>
#include <iomanip>
#include <fstream>
#include <list>
#include <string>
#include <vector>
#include <sstream>
#include <set>
#include <queue>
#include <stack>
#include <map>
#include <cmath>
// hey my dear c++03 compiler how are you
#include <functional>
//#include <random>

#define range(a) (a).begin(),(a).end()
#define rrange(a) (a).rbegin(),(a).rend()
typedef long long long64;
typedef long long int64;
using namespace std;

inline double sqr(double a) { return a * a; }





int main()
{
    freopen("input.txt", "r", stdin);
    int n;
    string s;
    cin >> n >> s;
    int64 ans = 0;
    int64 la=n - 1, ff=0;
    for (int64 i = 0; i < n; i++)
    {

        for (; la >= 0; la--)
        {
            if (s[la] == 'A')
                break;
        }
        for (; ff<n; ff++)
        {
            if (s[ff] == 'F')
                break;
        }

        if (ff > la)
        {
            break;
        }

        for (int64 f=ff; f<la; f++)
        {
            swap(s[f], s[f+1]);
            ans++;
        }

        la--;
        la--;
    }
    cout << ans;
    return 0;
}