#include <bits/stdc++.h>
using namespace std;
#define int long long
const int N = 2e5;
int n, k, prefix[3][N + 5];
signed main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
cin >> n >> k;
string s;
cin >> s;
while (s.size() && s.back() != 'I') s.pop_back();
reverse(s.begin(), s.end());
while (s.size() && s.back() != 'J') s.pop_back();
reverse(s.begin(), s.end());
n = s.size();
for (int i = 1; i <= n; i++) {
int v = s[i - 1] == 'J' ? 0 : (s[i - 1] == 'O' ? 1 : 2);
prefix[v][i]++;
for (int j = 0; j < 3; j++) prefix[j][i] += prefix[j][i - 1];
}
int ans = 1e18;
for (int i = 1; i <= n; i++) {
int l = 1, r = i, lefO = -1;
while(l <= r) {
int m = (l + r)/2;
if (prefix[1][i] - prefix[1][m - 1] >= k) lefO = m, l = m + 1;
else r = m - 1;
}
if (lefO == -1) continue;
l = 1, r = lefO - 1;
int lefJ = -1;
while (l <= r) {
int m = (l + r)/2;
if (prefix[0][lefO - 1] - prefix[0][m - 1] >= k) lefJ = m, l = m + 1;
else r = m - 1;
}
if (lefJ == -1) continue;
l = i + 1, r = n;
int rigI = -1;
while (l <= r) {
int m = (l + r)/2;
if (prefix[2][m] - prefix[2][i] >= k) rigI = m, r = m - 1;
else l = m + 1;
}
if (rigI == -1) continue;
ans = min(ans, rigI - lefJ + 1 - 3*k);
}
cout << (ans == 1e18 ? -1 : ans) << '\n';
return 0;
}
