#include <bits/stdc++.h>
// #pragma GCC optimize("Ofast,no-stack-protector,unroll-loops")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
// #define int long long
#define ld long double
#define ll long long
#define pii pair<int, int>
#define all(v) v.begin(), v.end()
using namespace std;
const int oo = 1e9 + 9;
const int MAX = 1e5 + 5, LOGMAX = 20, B = 441, MOD = 998244353;
void solve(){
int n, k; cin >> n >> k;
string s; cin >> s;
s = "S" + s;
queue<int> q[3];
map<char, int> mp = {{'J', 0}, {'O', 1}, {'I', 2}};
int nxt[n + 1][3];
memset(nxt, 0, sizeof(nxt));
int ans = oo;
for(int i = 1; i <= n; i++){
q[mp[s[i]]].push(i);
for(int j = 0; j < 3; j++){
if(q[j].size() > k) q[j].pop();
if(q[j].size() == k) nxt[i][j] = q[j].front();
}
int j = nxt[nxt[nxt[i][2]][1]][0];
if(j != 0) ans = min(ans, (i - j + 1) - 3 * k);
}
if(ans == oo) ans = -1;
cout << ans << '\n';
}
signed main(){
ios::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int t = 1;
// cin >> t;
while(t--) solve();
}
