#include <iostream>
#include <set>
#include <string>
#include <sstream>
#include <limits>
using namespace std;
class Jjooii {
public:
static void main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int N, K;
cin >> N >> K;
set<int> Js, Is, Os;
string S;
cin >> S;
for (int i = 0; i < N; i++) {
char c = S[i];
if (c == 'J') Js.insert(i);
else if (c == 'I') Is.insert(i);
else Os.insert(i);
}
int ans = numeric_limits<int>::max();
int iterations = 0;
if (!Js.empty()) {
int right = -1;
int left = *Js.begin();
Js.erase(Js.begin());
int nLeft = next(Js, K - 1, left);
if (nLeft >= 0) {
int nnLeft = next(Os, K, nLeft);
if (nnLeft > 0) {
right = next(Is, K, nnLeft);
}
}
if (right > 0) ans = min(ans, right - left + 1 - (3 * K));
while (right > 0 && !Js.empty() && iterations < 50000) {
left = *Js.begin();
Js.erase(Js.begin());
right = -1;
int nL = next(Js, K - 1, left);
if (nL >= 0) {
int nnL = next(Os, K, nL);
if (nnL > 0) {
right = next(Is, K, nnL);
}
}
if (right > 0) ans = min(ans, right - left + 1 - (3 * K));
iterations++;
}
}
if (ans == numeric_limits<int>::max()) ans = -1;
cout << ans << endl;
}
static int next(set<int>& s, int n, int curr) {
int counter = min(n, 1000);
int toReturn = curr;
auto it = s.upper_bound(toReturn);
while (counter > 0 && it != s.end()) {
toReturn = *it;
it++;
counter--;
}
if (counter > 0) return -1;
return toReturn;
}
};
int main() {
Jjooii::main();
return 0;
}
