#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define int ll
using P = pair<int, int>;
#define all(x) x.begin(), x.end()
#define rep(i, l, n) for (int i = l; i < (n); ++i)
#define sz(x) (int)x.size()
const char nl = '\n';
const int mod = 998244353;
const int inf = 1e9;
int f(char c) {
if (c == 'J')return 0;
if (c == 'O')return 1;
return 2;
}
void solve() {
int n, k; cin >> n >> k;
string s; cin >> s;
int p[n][3], sf[n+1][3];
memset(p, 0, sizeof p);
memset(sf, 0, sizeof sf);
vector<int> g[3];
rep(i, 0, n)
g[f(s[i])].push_back(i);
rep(i, 0, n) {
p[i][f(s[i])] += 1;
if (i)rep(j, 0, 3)p[i][j] += p[i-1][j];
}
for (int i = n-1; i >= 0; --i) {
sf[i][f(s[i])] += 1;
if (i < n-1)rep(j, 0, 3)sf[i][j] += sf[i+1][j];
}
int last = 0, ok= 1, res = inf;
if (sz(g[0]) < k)last = n-1, ok = 0;
else last = g[0][k-1];
rep(j, 1, 3) {
if (sz(g[j]) < p[last][j]+k){ok = 0;break;}
last = g[j][p[last][j]+k-1];
}
if (ok)res = last+1-3*k;
rep(i, 0, n) { // ilki i sany ayyryan.
ok = true;
rep(j, 0, 3)
ok &= (p[i][j] <= sf[0][j]-k);
if (!ok)break;
last = i, ok = 1;
rep(j, 0, 3) {
if (sz(g[j]) < p[last][j]+k){ok = 0; break;}
last = g[j][p[last][j]+k-1];
}
if (ok)res = min(res, last-i-3*k);
}
if (res == inf)res = -1;
cout << res << nl;
}
int32_t main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
solve();
return 0;
}
