#include <bits/stdc++.h>
// Author: Kazuki_Will_Win_VOI_8703
#define fi first
#define se second
#define pii pair<int, int>
#define int long long
#define all(a) a.begin(), a.end()
using namespace std;
const int mn = 2e5 + 5, bm = (1 << 11) + 1, mod = 1e9 + 7, offset = 5e4, B = 320 + 5;
const int inf = 1e18, base = 311;
string s;
int n, k, prefix[3][mn];
int right_min(int Megumi, int st){
int l = st, r = n, ans = -1;
while(l <= r){
int mid = (l + r) >> 1;
if(prefix[Megumi][mid] - prefix[Megumi][st - 1] >= k){
r = mid - 1;
ans = mid;
}
else{
l = mid + 1;
}
}
return ans;
}
void solve(){
cin >> n >> k >> s;
s = "#" + s;
for(int i = 1; i <= n; i++){
int add0 = 0, add1 = 0, add2 = 0;
if(s[i] == 'J') add0 ++;
if(s[i] == 'O') add1 ++;
if(s[i] == 'I') add2 ++;
prefix[0][i] = prefix[0][i - 1] + add0;
prefix[1][i] = prefix[1][i - 1] + add1;
prefix[2][i] = prefix[2][i - 1] + add2;
}
int res = inf;
for(int i = 1; i <= n; i++){
int l = i, r, cur = i, nxt;
//
nxt = right_min(0, cur);
if(nxt == -1) continue;
cur = nxt + 1;
nxt = right_min(1, cur);
if(nxt == -1) continue;
cur = nxt + 1;
nxt = right_min(2, cur);
if(nxt == -1) continue;
r = nxt;
res = min(res, r - l + 1 - 3 * k);
}
if(res == inf) res = -1;
cout << res << '\n';
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
int t = 1;
// cin >> t;
while(t--){
solve();
}
}
