#include <algorithm>
#include <array>
#include <iostream>
#include <string>
#include <vector>
int main() {
std::ios_base::sync_with_stdio(false);
std::cin.tie(0);
int n, k;
std::cin >> n >> k;
std::string s;
std::cin >> s;
for (int i = 0; i < n; ++i) {
if (s[i] == 'J') {
s[i] = 0;
} else if (s[i] == 'O') {
s[i] = 1;
} else if (s[i] == 'I') {
s[i] = 2;
}
}
std::vector<std::array<int, 3>> pref(n + 1, std::array<int, 3>{});
for (int i = 0; i < n; ++i) {
pref[i + 1] = pref[i];
pref[i + 1][s[i]]++;
}
int ans = n + 1;
for (int i = 0; i < n; ++i) {
bool ok = true;
int p = i;
for (int j = 0; j < 3; ++j) {
int r = n + 1;
for (int d = 20; d >= 0; --d) {
if (r - (1 << d) <= p) continue;
if (pref[r - (1 << d)][j] - pref[p][j] >= k) r -= (1 << d);
}
if (r > n) {
ok = false;
break;
}
p = r;
}
if (!ok) break;
int res = 0;
for (int j = 0; j < 3; ++j) res += pref[p][j] - pref[i][j] - k;
ans = std::min(ans, res);
}
if (ans == n + 1) ans = -1;
std::cout << ans << "\n";
return 0;
}
