#include <bits/stdc++.h>
#define pb push_back
#define ll long long int
#define all(v) (v).begin(),(v).end()
#define fi first
#define se second
using namespace std;
int main() {
int n, k; string s;
cin >> n >> k >> s;
vector<int> ji, ii, po(n+1);
for (int i = 1; i <= n; i++) {
if (s[i-1] == 'J') ji.pb(i);
if (s[i-1] == 'I') ii.pb(i);
po[i] = po[i-1];
if (s[i-1] == 'O') po[i]++;
}
int L = 0, R = 0; int mn = 1000000;
while (L+k-1 < ji.size() && R+k-1 < ii.size()) {
while (R+k-1 < ii.size() && (ji[L+k-1] > ii[R] || po[ii[R]]-po[ji[L+k-1]-1] < k)) {
R++;
}
if (R+k-1 < ii.size() && !(ji[L+k-1] > ii[R] || po[ii[R]]-po[ji[L+k-1]-1] < k)) {
mn = min(mn, ii[R+k-1]-ji[L]+1-3*k);
}
L++;
}
if (mn == 1000000) mn = -1;
cout << mn;
return 0;
}
