#include<bits/stdc++.h>
using namespace std;

#define ll long long
#define foru(a,b,c) for(ll a=b;a<=c;a++)
#define endl '\n'

ll n,m,k;
string s;
vector<ll>ma,fa;

int main() {
	freopen("Toilets.in","r",stdin);
	freopen("Toilets.out","w",stdout);
	ios::sync_with_stdio(0);
	cin.tie(0);
	cin>>n>>m>>s>>k;
	ll cnt=1;
	foru(i,0,k-1){
	    for(auto c:s){
	        if(c=='M')ma.push_back(cnt);
	        else fa.push_back(cnt);
	        cnt++;
        }
    }
    if(ma.size()>n){
        cout<<-1<<endl;
        return 0;
    }
    ll ans=0;
    foru(i,1,ma.size()){
        ans=max(ans,2LL*i-1-ma[i-1]);
    }
    ll fasize=fa.size();
    foru(i,1,n){
        ans=max(ans,2LL*i-1-fa[fasize-n+i-1]);
    }
    cout<<max(0LL,ans);
	return 0;
}