#include <bits/stdc++.h>
using namespace std;
#define int long long
#define endl '\n';
#define rep(n) for(int i = 0;i < (n);i++)
int a,b;
string S;
signed main()
{
    ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
    cin >> a >> b >> S;
    if(a + b > S.size())
    {
        cout << "-1" << endl;
    }else if(a + b <= S.size())
    {
        int s = 0;
        int t = 0;
        for(int i = 0;i < S.size();i++)
        {
            if(S[i] == 'S')
            {
                s++;
            }else if(S[i] == 'T')
            {
                t++;
            }
        }
    }
    return 0;
}