#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
#define INF 2000000000
string s2 = "", s = "";
vector<int> v[3];
int zerosize, onesize, twosize, J = 0, O = 0, I = 0, JO = 0, OI = 0, n, k, res = INF, p0 = 0, p1 = 0, p2 = 0; ;
void solve() {
bool notfirst = false;
for (int i = 1; i < k; i++) {
J += v[0][i] - v[0][i-1] - 1;
O += v[1][i] - v[1][i-1] - 1;
I += v[2][i] - v[2][i-1] - 1;
}
if (v[1][0] > v[0][k-1]) JO = v[1][0] - v[0][k-1] - 1;
if (v[2][0] > v[1][k-1]) OI = v[2][0] - v[1][k-1] - 1;
while (p0 + k <= zerosize && p1 + k <= onesize && p2 + k <= twosize) {
//find J
if (notfirst) J += v[0][p0+k-1] - v[0][p0+k-2] - (v[0][p0] - v[0][p0-1]);
// find O
if (v[1][p1] < v[0][p0+k-1]) {
while (p1 + k <= onesize && v[1][p1] < v[0][p0+k-1]) {
if (p1 + k == onesize) return;
O -= v[1][p1+1] - v[1][p1] - 1;
O += v[1][p1+k] - v[1][p1+k-1] - 1;
p1++;
}
}
/*if (notfirst) */JO = v[1][p1] - v[0][p0+k-1] - 1;
// find I
if (v[2][p2] < v[1][p1+k-1]) {
while (p2 + k <= twosize && v[2][p2] < v[1][p1+k-1]) {
if (p2 + k == twosize) return;
I -= v[2][p2+1] - v[2][p2] - 1;
I += v[2][p2+k] - v[2][p2+k-1] - 1;
p2++;
}
}
/*if (notfirst) */OI = v[2][p2] - v[1][p1+k-1] - 1;
res = min(res, J + O + I + JO + OI);
//cout << p0 << " " << p1 << " " << p2 << "--" << J << " " << O << " " << I << " " << JO << " " << OI << endl;
p0++;
notfirst = true;
}
}
int main() {
cin >> n >> k;
bool jyet = false, iyet = false;
for (int i = 0; i < n; i++) {
char a;
cin >> a;
if (a == 'J') jyet = true;
if (jyet) s2 += a;
}
for (int i = s2.length() - 1; i >= 0; i--) {
if (s2[i] == 'I') iyet = true;
if (iyet) s += s2[i];
}
reverse(s.begin(), s.end());
n = s.length();
for (int i = 0; i < n; i++) {
if (s[i] == 'J') v[0].push_back(i);
else if (s[i] == 'O') v[1].push_back(i);
else v[2].push_back(i);
}
zerosize = v[0].size(), onesize = v[1].size(), twosize = v[2].size();
// cout << "bef" << zerosize << " " << onesize << " " << twosize << endl;
if (n == 0 || zerosize < k || onesize < k || twosize < k) {
cout << -1;
return 0;
}
solve();
if (res == INF) cout << -1;
else cout << res;
}
// 13 2 JOJIOIOOJOJII
