#include <bits/stdc++.h>
#pragma GCC target("avx2")
#pragma GCC optimize("O3,unroll-loops")
using namespace std;
#define int long long
#define pii pair<int,int>
#define ff first
#define ss second
#define sp << " " <<
#define all(cont) cont.begin(),cont.end()
#define vi vector<int>
const int inf = 1e18,N = 3e5+1,MOD = 998244353,B = 250;
void solve() {
int n,k;
cin >> n >> k;
string s;
cin >> s;
vi js(n+1,0),os(n+1,0),is(n+1,0);
for (int i=1;i<=n;i++) {
js[i] = js[i-1]+(s[i-1] == 'J');
os[i] = os[i-1]+(s[i-1] == 'O');
is[i] = is[i-1]+(s[i-1] == 'I');
}
int ans = inf;
for (int i=1;i<=n;i++) {
int p = i;
int l = 1,r = n;
while (l<=r) {
int m = (l+r) >> 1;
if (js[m]-js[p-1] < k) l = m+1;
else r = m-1;
}
if (l > n) continue;
p = l;
l = p+1;
r = n;
while (l<=r) {
int m = (l+r) >> 1;
if (os[m]-os[p-1] < k) l = m+1;
else r = m-1;
}
if (l > n) continue;
p = l;
l = p+1;
r = n;
while (l <= r) {
int m = (l+r) >> 1;
if (is[m]-is[p-1] < k) l = m+1;
else r = m-1;
}
if (l > n) continue;
ans = min(ans,l-i+1-3*k);
}
if (ans < inf) cout << ans << '\n';
else cout << -1 << '\n';
}
int32_t main() {
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#ifdef Dodi
freopen("in.txt","r",stdin);
freopen("out.txt","w",stdout);
#endif
int t = 1;
//cin >> t;
while (t --> 0) solve();
}
