#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ull unsigned long long
#define pb(e) push_back(e)
#define sv(a) sort(a.begin(),a.end())
#define sa(a,n) sort(a,a+n)
#define mp(a,b) make_pair(a,b)
#define vf first
#define vs second
#define ar array
#define all(x) x.begin(),x.end()
const int inf = 0x3f3f3f3f;
const int mod = 1000000007;
const double PI=3.14159265358979323846264338327950288419716939937510582097494459230;
bool remender(ll a , ll b){return a%b;}
void solve(){
int n , k;
cin >> n >> k;
string s;
cin >> s;
int cnt = 0;
char c = 'J';
vector<int> v[3];
for(int i = 0; i < n; i++){
if(s[i] == 'J')v[0].pb(i);
else if(s[i] == 'O')v[1].pb(i);
else v[2].pb(i);
if(c == s[i] && c != 'a'){
cnt++;
if(cnt == k){
cnt = 0;
if(c == 'I'){
c = 'a';
}
if(c == 'O')c = 'I';
if(c == 'J')c = 'O';
}
}
}
if(c == 'a'){
int ans = 1e9;
for(int i = 0; i+(k-1) < (int)v[0].size(); i++){
int last = v[0][i + (k-1)];
int p = upper_bound(all(v[1]) , last) - v[1].begin();
if(p + (k - 1) < (int)v[1].size()){
p = upper_bound(all(v[2]) , v[1][p + (k-1)]) - v[2].begin();
if(p + (k-1) < (int)v[2].size()){
last = v[2][p + (k-1)];
ans = min(ans , (last - v[0][i] + 1) - 3 * k);
}
}
}
cout << ans << '\n';
}
else {
cout << -1 << '\n';
}
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
//int t;cin >> t;while(t--)
solve();
return 0;
}
