#include <bits/stdc++.h>
using namespace std;
#define ll long long
const ll nx=2e5+5;
ll n, m, t, sf[nx], skip, cur, ans;
string s;
queue<int> q;
int main()
{
cin.tie(NULL)->sync_with_stdio(false);
cin>>n>>m>>s>>t;
n=2*n;
for (int i=n-1; i>=0; i--)
{
sf[i]=(sf[i+1]+(s[i]=='M'));
if (s[i]=='F') q.push(i);
}
if (sf[0]>n/2) return cout<<-1, 0;
for (int i=n-1; i>=0; i--)
{
if (s[i]=='M')
{
if (cur-1<-1)
{
ans=max(ans, sf[q.front()]-skip);
}
else cur--, skip++;
}
else
{
if (q.front()==i) q.pop(), cur++;
}
}
cout<<ans;
}
