#include <bits/stdc++.h>
using namespace std;
#define fastio ios::sync_with_stdio(0); cin.tie(0); cout.tie(0)
#define ll long long
#define pii pair<int, int>
#define pll pair<ll, ll>
#define pb push_back
#define fi first
#define se second
ll n, k, ans, j, o, i, cntj, cnto, cnti;
ll a[101];
string joi; //ironically, I'm not joiful.
int main() {
fastio;
cin >> n >> k >> joi;
ans = 1e18;
for (int t = 0; t < n; t++) {
while (j < n && cntj < k) {
if (joi[j] == 'J') {
cntj++;
}
if (joi[j] == 'O') {
cnto--;
}
j++;
}
while (o < n && cnto < k) {
if (joi[o] == 'O') {
cnto++;
}
if (joi[o] == 'I') {
cnti--;
}
o++;
}
while (i < n && cnti < k) {
if (joi[i] == 'I') {
cnti++;
}
i++;
}
if (cnti != k) {
break;
}
ans = min(ans, i - t - 3*k);
if (joi[t] == 'J') {
cntj--;
}
}
if (ans < n) {
cout << ans << endl;
} else {
cout << -1 << endl;
}
}
