#include <bits/stdc++.h>
#define INF 0x3f3f3f3f
using namespace std;
const int kN = 2e5;
const int kLog = 17;
int n, k, nxt[3][1 + kN][1 + kLog], nxt2[3][1 + kN];
string s;
void minSelf(int &x, int y) {
if (y < x) {
x = y;
}
}
void build(int t) {
nxt[t][n][0] = n;
nxt2[t][n] = n;
for (int i = n - 1; i >= 0; --i) {
if (i + 1 < n && s[i + 1] - '0' == t) {
nxt[t][i][0] = i + 1;
} else {
nxt[t][i][0] = nxt[t][i + 1][0];
}
if (s[i] - '0' == t) {
nxt2[t][i] = i;
} else {
nxt2[t][i] = nxt2[t][i + 1];
}
}
for (int i = 1; i <= kLog; ++i) {
nxt[t][n][i] = n;
for (int j = 0; j < n; ++j) {
if (nxt[t][j][i - 1] == n) {
nxt[t][j][i] = n;
} else {
nxt[t][j][i] = nxt[t][nxt[t][j][i - 1]][i - 1];
}
}
}
}
void lift(int t, int &index) {
for (int i = kLog; i >= 0; --i) {
if (k & (1 << i)) {
index = nxt[t][index][i];
}
}
}
void testCase() {
cin >> n >> k >> s;
k -= 1;
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';
}
}
for (int i = 0; i < 3; ++i) {
build(i);
}
int ans = INF;
for (int i = 0; i < n; ++i) {
if (s[i] == '0') {
int j = i;
for (int p = 0; p < 3; ++p) {
lift(p, j);
if (p + 1 < 3) {
j = nxt2[p + 1][j];
}
}
if (j < n) {
minSelf(ans, j - i + 1 - 3 * (k + 1));
}
}
}
if (ans == INF) {
cout << "-1\n";
} else {
cout << ans << '\n';
}
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int tests = 1;
for (int tc = 0; tc < tests; ++tc) {
testCase();
}
return 0;
}
