#include "bits/stdc++.h"
using namespace std;
#define SZ(s) (int)s.size()
const int N = 2e5 + 5;
#define ll long long
int main() {
ios_base::sync_with_stdio(false); cin.tie(nullptr);
int n, k;
string s;
cin >> n >> k >> s;
vector <vector <int>> v(3), a(3);
for(int i = 0; i < n; i++) {
if(s[i] == 'J') v[0].push_back(i);
if(s[i] == 'O') v[1].push_back(i);
if(s[i] == 'I') v[2].push_back(i);
}
for(int i = 0; i < 3; i++) {
a[i].push_back(0);
for(int j = 0; j < SZ(v[i])-1; j++) {
a[i].push_back(a[i].back() + (v[i][j+1] - v[i][j] - 1));
}
}
int ans = 1e9;
for(int i = 0; i + k - 1 < SZ(v[0]); i++) {
int s = a[0][i + k - 1] - a[0][i];
int x = (v[0][i + k - 1] + 1);
int t = lower_bound(v[1].begin(), v[1].end(), v[0][i + k - 1]) - v[1].begin();
if(t + k - 1 >= SZ(v[1])) break;
int y = (v[1][t]);
s += (y - x);
s += a[1][t + k - 1] - a[1][t];
x = (v[1][t + k - 1] + 1);
t = lower_bound(v[2].begin(), v[2].end(), v[1][t + k - 1]) - v[2].begin();
if(t + k - 1 >= SZ(v[2])) break;
y = (v[2][t]);
s += (y - x);
s += a[2][t + k - 1] - a[2][t];
ans = min(ans, s);
}
cout << (ans == 1e9 ? -1 : ans);
return 0;
}
