#include <bits/stdc++.h>
using namespace std;
#ifdef MIKU
string dbmc = "\033[1;38;2;57;197;187m", dbrs = "\033[0m";
#define debug(x...) cout << dbmc << "[" << #x << "]: ", dout(x)
void dout() { cout << dbrs << endl; }
template <typename T, typename ...U>
void dout(T t, U ...u) { cout << t << (sizeof...(u) ? ", " : ""); dout(u...); }
#else
#define debug(...) 39
#endif
#define fs first
#define sc second
#define mp make_pair
#define FOR(i, j, k) for (int i = j, Z = k; i < Z; i++)
using ll = long long;
typedef pair<int, int> pii;
int n, m;
string s, k;
void miku() {
cin >> n >> m;
assert(m == 1);
cin >> s >> k;
reverse(s.begin(), s.end());
int now = 1, ans = 1;
for (auto &i : s) {
if (i == 'F') now++;
else {
now--;
ans = min(ans, now);
}
}
if (now < 0) cout << -1 << '\n';
else cout << max(0, -ans) << '\n';
}
int32_t main() {
cin.tie(0) -> sync_with_stdio(false);
cin.exceptions(cin.failbit);
miku();
return 0;
}
