#include <bits/stdc++.h>
using namespace std;
#define rep(i, a, b) for (auto i = (a); i < (b); ++i)
#define per(i, a, b) for (auto i = (b); i-- > (a); )
#define lb(x...) lower_bound(x)
#define ub(x...) upper_bound(x)
const int N = 1 << 18, inf = 1e9;
char s[N];
int ci[N], co[N], cj[N];
signed main() {
cin.tie(nullptr)->sync_with_stdio(false);
int n, k, res = inf; cin >> n >> k >> s;
for (int l = 0, j = 0, o = 0, i = 0, cj = 0, co = 0, ci = 0; l < n; ++l) {
while (j < n && cj < k) {
if (s[j] == 'J') ++cj;
if (s[j] == 'O') --co;
++j;
}
while (o < n && co < k) {
if (s[o] == 'O') ++co;
if (s[o] == 'I') --ci;
++o;
}
while (i < n && ci < k) if (s[i++] == 'I') ++ci;
if (ci != k) break;
res = min(res, i - l - 3 * k);
if (s[l] == 'J') --cj;
}
cout << (res < inf ? res : -1);
}
