#include<bits/stdc++.h>
using namespace std;
#define fast ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
#define ll long long int
const ll N = 100+5 , INF = 1e18 , MOD = 1e9+7;
void solve(){
ll n,k;
string s;
cin >> n >> k >> s;
s = ' '+s;
vector<ll> nearj(n+5) , nearo(n+5),neari(n+5);
ll lastj = 0 , lasto = 0 , lasti = 0;
for(ll i = 1; i <= n; i++){
nearj[i] = lastj;
nearo[i] = lasto;
neari[i] = lasti;
if(s[i] == 'J') lastj = i;
else if(s[i] == 'O') lasto = i;
else lasti = i;
}
ll ans = INF;
for(ll i = 1; i <= n; i++){
if(s[i] == 'I'){
ll x = i;
for(ll j = 1; j < k; j++) x = neari[x];
x = nearo[x];
for(ll j = 1; j < k; j++) x = nearo[x];
x = nearj[x];
for(ll j = 1; j < k; j++) x = nearj[x];
if(x > 0) ans = min(ans,(i-x+1)-(3*k));
}
}
if(ans == INF) ans = -1;
cout << ans;
}
int main(){
fast;
ll tc = 1;
// cin >> tc;
while(tc--) solve();
return 0;
}
