/// @author Camillus
#include <bits/stdc++.h>

using ll = long long;
using namespace std;

signed main() {
#ifndef LOCAL
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
#else
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif
    int n, k;
    cin >> n >> k;
    
    string s;
    cin >> s;

    map<char, vector<int>> p;

    p['J'] = vector<int>(n + 1);
    p['O'] = vector<int>(n + 1);
    p['I'] = vector<int>(n + 1);

    for (int i = 1; i <= n; i++) {
        p['J'][i] = p['J'][i - 1];
        p['O'][i] = p['O'][i - 1];
        p['I'][i] = p['I'][i - 1];
        p[s[i - 1]][i] += 1;
    }

    int ans = INT32_MAX;

    auto findK = [&p, &k](int i, char x) -> int {
        if (i <= 0 || p[x][i] < k) {
            return 0;
        }
        int l = 0, r = i;
        while (r - l > 1) {
            int m = (l + r) / 2;
            if (p[x][i] - p[x][m] >= k) {
                l = m;
            } else {
                r = m;
            }
        }
        return l + 1;
    };

    for (int r = 1; r <= n; r++) {
        int x = r;
        x = findK(r, 'I');
        x = findK(x - 1, 'O');
        x = findK(x - 1, 'J');
        if (x) {
            ans = min(ans, r - x + 1 - 3 * k);
        }
    }

    if (ans == INT32_MAX) {
        cout << -1 << '\n';
    } else {
        cout << ans << '\n';
    }
    return 0;
}