#include <bits/stdc++.h>
using namespace std;
string w = "JOI";
int n, k;
string s;
vector<array<int, 3>> pf;
int pTem(int start, int b) {
int l = start, r = n, ans = n + 1;
while (l <= r) {
int m = (l+r)/2;
int qtd = (pf[m][b] - pf[start - 1][b]);
if (qtd >= k) {
ans = m;
r = m - 1;
} else {
l = m + 1;
}
}
return ans;
}
int INF = 0x3f3f3f3f;
int32_t main() {
//freopen("in", "r", stdin);
cin >> n >> k >> s;
pf.resize(n + 1);
for (int i = 1; i <= n; i++) {
pf[i] = pf[i - 1];
for (int j = 0; j < 3; j++) {
if (s[i - 1] == w[j]) pf[i][j]++;
}
}
int ans = INF;
for (int i = 1; i <= n; i++) {
int a = pTem(i, 0);
int b = pTem(a, 1);
int c = pTem(b, 2);
if (c == (n + 1)) continue;
ans = min(ans, c - i + 1 - 3*k);
}
if (ans == INF) cout << "-1\n";
else cout << ans << "\n";
}
