#include <cstdio>
#include <set>
#include <deque>
#include <iostream>
#include <string>
#include <stack>
#include <algorithm>
#include <cstring>
using namespace std;
#define N 200005
int sub1() {
int n, answer, perm[22];
char s[22], s0[22];
stack<int> sm,sf;
answer = 1e9;
scanf("%d%*d%s%*d", &n, s);
int count_m = count(s, s + n + n, 'M');
for (int i = 2 * n; i--; ) {
if (s[i] == 'M') sm.push(i);
else sf.push(i);
}
memcpy(s0, s, sizeof s);
for (int m = 0; m < (1 << (n << 1)); ++m) {
if (__builtin_popcount(m) != count_m)
continue;
memcpy(s, s0, sizeof s);
for (int j = 0; j < 2 * n; ++j) {
if ((m >> j) & 1)
s[j] = 'M';
else
s[j] = 'F';
}
int ok = 1;
for (int j = 0; j < 2 * n; j += 2) {
if (s[j] == s[j + 1] and s[j] == 'M') {
auto it = find(s + j, s + 2 * n, 'F');
if (it == 2 * n + s) {
ok = 0;
break;
}
rotate(s + j + 1, it, it + 1);
}
}
if (!ok)
continue;
auto sf_ = sf, sm_ = sm;
for (int j = 0; j < 2 * n; ++j) {
if ((m >> j) & 1)
perm[j] = sm_.top(), sm_.pop();
else
perm[j] = sf_.top(), sf_.pop();
}
int here = 0;
for (int i = 0; i < 2 * n; ++i) {
int x = 0;
for (int j = 0; j < i; ++j)
x += perm[j] > perm[i];
here = max(here, x);
}
answer = min(answer, here);
}
printf("%d\n", answer >= 1e9 ? -1 : answer);
exit(0);
return 0;
}
bool String_IsUsable(const string &s) {
int n = s.size() >>1;
int i_, i;
set<pair<long long, char> > st;
set<long long> sf;
for (i_ = 0; i_ < 2 * n; ++i_) {
st.insert(make_pair(i_ * 300000ll, s[i_]));
if (s[i_] == 'F')
sf.insert(i_ * 300000ll);
}
//printf(" Checking %s\n", s.c_str());
while (st.size()) {
auto x1 = *st.begin();
st.erase(st.begin());
auto x2 = *st.begin();
st.erase(st.begin());
if (x1.second == 'M' and x2.second == 'M') {
while (sf.size() && *sf.begin() < x2.first)
sf.erase(sf.begin());
if (sf.empty())
return false;
//printf(" Need help \n");
long long y = *sf.begin();
//printf(" %lld -> %lld\n", x2.first, y);
sf.erase(y);
st.insert(x2);
st.erase(make_pair(y, 'F'));
} else {
}
}
return true;
}
namespace sub2 {
int n, _;
string s0;
int check(int loose) {
string s;
deque<pair<int, int> > wom;
int miku = 0;
for (int i = 0; i < 2 * n; ++i) {
if (s0[i] == 'M') {
s += 'M';
++miku;
while (wom.size() and miku == wom[0].first) {
s += 'F';
wom.pop_front();
}
} else
wom.push_back(make_pair(miku + loose, i));
while (wom.size() and miku == wom[0].first) {
s += 'F';
wom.pop_front();
}
}
while (wom.size())
s += 'F', wom.pop_back();
//printf(" Loose = %d, built %s, usable = %d\n", loose,s.c_str(), (int)String_IsUsable(s));
return (String_IsUsable(s));
}
void Main() {
cin >> n >> _ >> s0 >> _;
int x = n;
for (int j = 1 << 19; j >>= 1;) {
if (x >= j && check(x - j))
x -= j;
}
if (x == n)
cout << "-1\n";
else
cout << x << '\n';
}
}
int main() {
sub2::Main();
}
