#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int ind[256];
const int inf = 1e9 + 5;
bool chmin(int &a, const int &b) {
return a > b ? a = b, true : false;
}
bool chmax(int &a, const int &b) {
return a < b ? a = b, true : false;
}
void solve() {
int n, k; cin >> n >> k;
string s; cin >> s;
ind['J'] = 0;
ind['O'] = 1;
ind['I'] = 2;
vector<array<int, 3>> pref(n + 1);
for(int i = 0; i < n; i++) {
pref[i + 1] = pref[i];
pref[i + 1][ind[s[i]]]++;
}
vector<array<int, 3>> ptr(n + 1);
for(int i = 1; i <= n; i++) {
ptr[i] = ptr[i - 1];
for(int j = 0; j < 3; j++) {
while(ptr[i][j] < n && pref[i][j] - pref[ ptr[i][j] ][j] >= k) {
ptr[i][j]++;
}
}
}
vector<array<int, 5>> mxp(n + 1, {-inf, -inf, -inf, -inf, -inf});
int ans = inf;
for(int i = 1; i <= n; i++) {
for(int j = 0; j < 5; j++) {
mxp[i][j] = mxp[i - 1][j];
}
if(s[i - 1] == 'J' && ptr[i][0] > 0) {
chmax(mxp[i][0], ptr[i][0]);
}
if(s[i - 1] == 'O') {
chmax(mxp[i][1], mxp[i][0]);
if(ptr[i][1] > 0) chmax(mxp[i][2], mxp[ ptr[i][1] ][1]);
}
if(s[i - 1] == 'I') {
chmax(mxp[i][3], mxp[i][2]);
if(ptr[i][2] > 0) chmax(mxp[i][4], mxp[ ptr[i][2] ][3]);
if(mxp[i][4] > 0) chmin(ans, i - mxp[i][4] + 1);
}
}
if(ans == inf) {
cout << -1 << '\n';
return;
}
cout << ans - 3 * k << '\n';
}
signed main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
solve();
}
