#ifndef DEBUG
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,avx,avx2,abm,mmx,fma,popcnt")
#endif
#include <bits/stdc++.h>
// #define int long long
using ll = long long;
using namespace std;
signed main() {
ios_base::sync_with_stdio(false);
cin.tie(0);
int n, k;
cin >> n >> k;
string s; cin >> s;
int lj = 0;
while (lj < n && s[lj] != 'J') lj++;
int rj = lj;
int j_cnt = 1;
int cur_cost = 0;
while ((rj+1)<n && j_cnt<k) {
rj++;
j_cnt += s[rj] == 'J';
// cur_cost += s[rj] != 'J';
}
int ro = rj;
int o_cnt = 0;
while ((ro+1)<n && o_cnt < k) {
ro++;
o_cnt += s[ro] == 'O';
// cur_cost += s[ro] != 'O';
}
int ri = ro;
int i_cnt = 0;
while ((ri+1)<n && i_cnt<k) {
ri++;
i_cnt += s[ri] == 'I';
}
if (j_cnt<k || o_cnt<k || i_cnt<k) {
cout << -1 << "\n";
return 0;
}
cur_cost = (ri-lj+1) - 3*k;
int res = cur_cost;
while (ri < n) {
rj++;
while (rj < n && s[rj] != 'J') {
// cur_cost += s[rj] != 'J';
o_cnt -= (s[rj]=='O' && rj<=ro);
i_cnt -= (s[rj]=='I' && rj > ro && rj<=ri);
rj++;
}
lj++;
while (lj<n && s[lj]!='J') {
// cur_cost -= s[lj] != 'J';
lj++;
}
ro = max(ro, rj);
while ((ro+1)<n && o_cnt<k) {
ro++;
o_cnt += s[ro]=='O';
i_cnt -= (s[ro]=='I' && ro<=ri);
}
ri = max(ri, ro);
while ((ri+1)<n && i_cnt<k) {
ri++;
i_cnt += s[ri]=='I';
}
if (rj>=n || ro>=n || ri >= n) break;
if (j_cnt<k || o_cnt<k || i_cnt<k) break;
cur_cost = (ri-lj+1) - 3*k;
res = min(res, cur_cost);
}
cout << res << "\n";
}
