#include <bits/stdc++.h>
using namespace std;
typedef long long lli;
#define f0(i,n) for(int32_t i = 0; i < (n); i++)
#define f1(i,n) for(int32_t i = 1; i <= (n); i++)
#define inp(n) lli n; cin >> n
#define inparr(arr,n) lli arr[n]; f0(t_ind, n) cin >> arr[t_ind]
#define all(x) (x).begin(), (x).end()
int main () {
inp(n);
inp(k);
string s;
cin >> s;
int nextkthJ[n];
int nextkthO[n];
int nextkthI[n];
memset(nextkthJ, -1, sizeof(nextkthJ));
memset(nextkthO, -1, sizeof(nextkthO));
memset(nextkthI, -1, sizeof(nextkthI));
int l=0, r=0, cnt = 0;
while (r < n) {
if (s[r] == 'J') {
cnt++;
}
if (cnt == k) {
while(s[l] != 'J') {
nextkthJ[l] = r;
l++;
}
nextkthJ[l] = r;
l++;
cnt--;
}
r++;
}
l=0, r=0, cnt = 0;
while (r < n) {
if (s[r] == 'O') {
cnt++;
}
if (cnt == k) {
while(s[l] != 'O') {
nextkthO[l] = r;
l++;
}
nextkthO[l] = r;
l++;
cnt--;
}
r++;
}
l=0, r=0, cnt = 0;
while (r < n) {
if (s[r] == 'I') {
cnt++;
}
if (cnt == k) {
while(s[l] != 'I') {
nextkthI[l] = r;
l++;
}
nextkthI[l] = r;
l++;
cnt--;
}
r++;
}
lli ans = 1e9;
f0(i, n) {
if(nextkthJ[i] == -1) break;
if(nextkthO[nextkthJ[i]] == -1) break;
if(nextkthI[nextkthO[nextkthJ[i]]] == -1) break;
ans = min(ans, (lli)nextkthI[nextkthO[nextkthJ[i]]] - i + 1);
}
if(ans == 1e9) cout << -1 << endl;
else cout << ans - 3*k << endl;
return 0;
}
