//#include "gondola.h"
#include <bits/stdc++.h>
#define pb push_back
#define pf push_front
using namespace std;
#define F first
#define S second
typedef long long ll;
#define pii pair <int, int>
#define pll pair <ll, ll>
typedef long double ld;
const ll N = 2 * 1e5 + 100, M = 1e7 + 10, len = 21, inf = 1e18;
const ll mod = 1000000009LL;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
ll um(ll a, ll b){
return (1LL * a * b) % mod;
}
ll subr(ll a, ll b){
return ((1LL * a - b) % mod + mod) % mod;
}
int main() {
ios::sync_with_stdio(false);
cin.tie(NULL);
int n, k, ans = INT_MAX;
cin >> n >> k;
string s;
cin >> s;
deque <int> v1, v2, v3;
for(int i = 0; i < n; i++){
if(s[i] == 'J') v1.pb(i);
if(s[i] == 'O') v2.pb(i);
if(s[i] == 'I') v3.pb(i);
}
while((int)v1.size() >= k){
while((int)v2.size() > 0){
if(v2[0] < v1[k - 1]) v2.pop_front();
else break;
}
if((int)v2.size() < k) break;
while((int)v3.size() > 0){
if(v3[0] < v2[k - 1]) v3.pop_front();
else break;
}
if((int)v3.size() < k) break;
ans = min(ans, v3[k - 1] - v1[0] + 1 - 3 * k);
v1.pop_front();
}
if(ans == INT_MAX) cout << -1;
else cout << ans;
return 0;
}
