#include <bits/stdc++.h>
using namespace std;
#define fast ios::sync_with_stdio(0);cin.tie(0);
#define s second
#define f first
typedef long long ll;
const ll MOD = 998244353;
const ll LOGN = 21;
const ll MAXN = 2e5 + 100;
string s;
int toJ[MAXN], toO[MAXN], toI[MAXN];
int main() {
fast
int N, K;
cin >> N >> K >> s;
s = "#" + s;
int r = 1;
int cnt = (s[1] == 'J');
for (int l = 1; l <= N; l++) {
while (cnt < K && r < N) {
r++;
if (s[r] == 'J')
cnt++;
}
if (cnt == K)
toJ[l] = r + 1;
else
toJ[l] = -1;
if (s[l] == 'J')
cnt--;
}
r = 1;
cnt = (s[1] == 'O');
for (int l = 1; l <= N; l++) {
while (cnt < K && r < N) {
r++;
if (s[r] == 'O')
cnt++;
}
if (cnt == K)
toO[l] = r + 1;
else
toO[l] = -1;
if (s[l] == 'O')
cnt--;
}
r = 1;
cnt = (s[1] == 'I');
for (int l = 1; l <= N; l++) {
while (cnt < K && r < N) {
r++;
if (s[r] == 'I')
cnt++;
}
if (cnt == K)
toI[l] = r + 1;
else
toI[l] = -1;
if (s[l] == 'I')
cnt--;
}
int ans = 1e9;
for (int i = 1; i <= N; i++) {
int now = toJ[i];
if (now == -1 || now > N)
continue;
now = toO[now];
if (now == -1 || now > N)
continue;
now = toI[now];
if (now == -1)
continue;
ans = min(ans, now - i);
}
if (ans == 1e9)
cout << "-1\n";
else
cout << ans - 3*K << "\n";
}
