#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define inf INT_MAX
#define f(i,k,n) for(int i=k; i<n; i++)

void solve()
{
    int n;
    cin>>n;
    int k=0;
    char x;
    f(i,0,n/2)
    {
        cin>>x;
        if(x=='A')  k++;
    }
    cout<<k*2<<"\n";
}

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    int t=1;
    //cin>>t;
    while(t--)  solve();
    return 0;
}