#include <bits/stdc++.h>
typedef long long ll;
using namespace std;
#define f first
#define s second
#define pb push_back
#define ep emplace
#define eb emplace_back
#define lb lower_bound
#define ub upper_bound
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define uniquev(v) sort(all(v)), (v).resize(unique(all(v)) - (v).begin())
#define mem(f,x) memset(f , x , sizeof(f))
#define sz(x) (int)(x).size()
#define __lcm(a, b) (1ll * ((a) / __gcd((a), (b))) * (b))
#define mxx *max_element
#define mnn *min_element
#define cntbit(x) __builtin_popcountll(x)
#define len(x) (int)(x.length())
const int N = 2e5 + 100;
int pr[N], suff[N];
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
vector <int> pos;
deque <int> dq;
int n, k;
cin >> n >> k;
string s;
cin >> s;
s = 'a' + s;
for (int i = 1; i <= n; i++) {
if (s[i] == 'O')
pos.pb(i);
pr[i] = -1;
if (s[i] == 'J') {
dq.push_back(i);
}
if (sz(dq) > k) {
dq.pop_front();
}
if (sz(dq) == k) {
pr[i] = dq.front();
}
}
while (sz(dq))
dq.pop_back();
for (int i = n; i >= 1; i--) {
suff[i] = -1;
if (s[i] == 'I') {
dq.push_back(i);
}
if (sz(dq) > k)
dq.pop_front();
if (sz(dq) == k) {
suff[i] = dq.front();
}
}
int ans = n + 1;
for (int i = k - 1; i < sz(pos); i++) {
int r = suff[pos[i]], l = pr[pos[i - k + 1]];
if (r != -1 && l != -1) {
ans = min(ans, r - l + 1 - k * 3);
}
}
if (ans == n + 1)
ans = -1;
cout << ans << '\n';
return 0;
}
