#include<bits/stdc++.h>
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
typedef long long ll;
typedef long double ld;
#define pb push_back
#define pii pair<int, int>
#define pll pair<ll, ll>
#define st first
#define nd second
#define ordered_set tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>
#define debug false
string s;
const int MAXN = 200 * 1000 + 17;
int sprefj[MAXN];
int sprefo[MAXN];
int sprefi[MAXN];
void dodaj (int i) {
if (s[i] == 'J') {
sprefj[i] ++;
}
if (s[i] == 'O') {
sprefo[i] ++;
}
if (s[i] == 'I') {
sprefi[i] ++;
}
}
int main () {
ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int n, K;
cin >> n >> K >> s;
s = "#" + s;
for (int i = 1; i <= n; i ++) {
sprefj[i] = sprefj[i - 1];
sprefo[i] = sprefo[i - 1];
sprefi[i] = sprefi[i - 1];
dodaj(i);
//cout << sprefj[i] << "\n";
}
sprefj[n + 1] = INT_MAX;
sprefo[n + 1] = INT_MAX;
sprefi[n + 1] = INT_MAX;
int wyn = INT_MAX;
for (int i = 1; i <= n; i ++) {
int p = 0, k = n + 1, wj;
while (p <= k) {
int sr = (p + k)/ 2;
if (sprefj[sr] >= sprefj[i - 1] + K) {
wj = sr;
k = sr - 1;
} else {
p = sr + 1;
}
}
p = 0, k = n + 1;
int wo;
while (p <= k) {
int sr = (p + k)/ 2;
if (sprefo[sr] >= sprefo[wj - 1] + K) {
wo = sr;
k = sr - 1;
} else {
p = sr + 1;
}
}
p = 0, k = n + 1;
int wi;
while (p <= k) {
int sr = (p + k)/ 2;
if (sprefi[sr] >= sprefi[wo - 1] + K) {
wi = sr;
k = sr - 1;
} else {
p = sr + 1;
}
}
//cout << wj << " " << wo << " " << wi << "\n";
if (wi != n + 1) {
wyn = min(wyn, wi - i + 1 - K * 3);
}
}
if (wyn == INT_MAX) {
cout << -1 << "\n";
} else {
cout << wyn << "\n";
}
return 0;
}
