//#pragma GCC optimize("Ofast")
//#pragma GCC target("avx,avx2,fma")
#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ull unsigned long long
#define db long double
#define pb push_back
#define ppb pop_back
#define fi first
#define se second
#define mp make_pair
#define all(x) (x).begin(), (x).end()
void dout() { cerr << '\n'; }
template <typename Head, typename... Tail>
void dout(Head H, Tail... T) {
cerr << " " << H;
dout(T...);
}
#ifdef LOCAL
#define dbg(...) cerr << #__VA_ARGS__, dout(__VA_ARGS__)
#else
#define dbg(...) ;
#endif
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
typedef pair <int, int> pii;
const int N = 2e5 + 123;
int n, k, p[3][N], sf[3][N];
int l[3][N], r[3][N];
int main() {
ios_base::sync_with_stdio(false), cin.tie(NULL);
#ifdef LOCAL
freopen("input.txt", "r", stdin);
#endif
string s;
cin >> n >> k >> s;
for (int i = 1; i <= n; i++) {
for (int j = 0; j < 3; j++) {
p[j][i] = p[j][i - 1];
}
if (s[i - 1] == 'J') {
p[0][i]++;
l[0][p[0][i]] = i;
}
if (s[i - 1] == 'O') {
p[1][i]++;
l[1][p[1][i]] = i;
}
if (s[i - 1] == 'I') {
p[2][i]++;
l[2][p[2][i]] = i;
}
}
for (int i = n; i >= 1; i--) {
for (int j = 0; j < 3; j++) {
sf[j][i] = sf[j][i + 1];
}
if (s[i - 1] == 'J') {
sf[0][i]++;
r[0][sf[0][i]] = i;
}
if (s[i - 1] == 'O') {
sf[1][i]++;
r[1][sf[1][i]] = i;
}
if (s[i - 1] == 'I') {
sf[2][i]++;
r[2][sf[2][i]] = i;
}
}
int j = 1, ans = N;
for (int i = 1; i <= n; i++) {
while (j < i && p[1][i] - p[1][j] >= k) {
j++;
}
if (s[i - 1] == 'O' && p[1][i] - p[1][j - 1] == k) {
if (r[0][sf[0][j] + k] && l[2][p[2][i] + k]) {
ans = min(ans, l[2][p[2][i] + k] - r[0][sf[0][j] + k]);
}
}
}
if (ans == N) {
cout << -1;
return 0;
}
cout << ans - 3 * k + 1;
}
