#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<ll, ll>;
const int MOD = 1e9+7;
vector<vector<int>> lift;
int jump(int n, int k) {
for (int i=0; i<21; i++) {
if (k&(1<<i)) {
n = lift[n][i];
}
}
return n;
}
#define cerr if(0) cerr
int main() {
ios::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
int n, k; cin >> n >> k;
string s; cin >> s;
lift.assign(n, vector<int>(21, 0));
vector<int> nxj(n, 0), nxo(n, 0), nxi(n, 0);
int nxtj=0, nxto=0, nxti=0;
for (int i=n-1; i>=0; i--) {
nxj[i] = nxtj;
nxi[i] = nxti;
nxo[i] = nxto;
if (s[i] == 'J') {
nxtj = i;
}
else if (s[i] == 'O') {
nxto = i;
}
else {
nxti = i;
}
}
for (int i=0; i<n; i++) {
if (s[i]=='J') {
lift[i][0] = nxj[i];
}
else if (s[i] == 'O') {
lift[i][0] = nxo[i];
}
else {
lift[i][0] = nxi[i];
}
}
for (int j=1; j<21; j++) {
for (int i=0; i<n; i++) {
lift[i][j] = lift[lift[i][j-1]][j-1];
}
}
int mn=1e9;
for (int i=0; i<n; i++) {
if (s[i] == 'J') {
cerr << i << ": " << endl;
int lastj = jump(i, k-1);
cerr << lastj << endl;
if (lastj < i) continue;
int frsto = nxo[lastj];
cerr << frsto << endl;
if (frsto < i) continue;
int lasto = jump(frsto, k-1);
cerr << lasto << endl;
if (lasto<i) continue;
int firsti = nxi[lasto];
cerr << firsti << endl;
if (firsti<i) continue;
int lasti = jump(firsti, k-1);
cerr << lasti << endl;
if (lasti<i) continue;
mn = min(mn, lasti-i+1-3*k);
}
}
cout << (mn==1e9 ? -1 : mn) << endl;
}
