#include <bits/stdc++.h>
using namespace std;
int main() {
ios::sync_with_stdio(0);
cin.tie(0), cout.tie(0);
int N, K;
cin >> N >> K;
string S;
cin >> S;
vector<vector<int>> P(N, vector<int>(3, 0));
for (int i = 0; i < N; i++) {
if (i > 0) {
P[i] = P[i - 1];
}
if (S[i] == 'J') {
P[i][0]++;
} else if (S[i] == 'O') {
P[i][1]++;
} else if (S[i] == 'I') {
P[i][2]++;
}
}
auto Sum = [&](int i, int j, int k) {
return P[j][k] - (i > 0 ? P[i - 1][k] : 0);
};
auto Next = [&](int s, int k) {
if (s == N) {
return N;
}
int lo = s, hi = N;
while (lo < hi) {
int md = (lo + hi) / 2;
if (Sum(s, md, k) >= K) {
hi = md;
} else {
lo = md + 1;
}
}
return lo;
};
auto Solve = [&](int st) {
int to = Next(Next(Next(st, 0), 1), 2);
if (to == N) {
return N + N;
}
int len = to - st + 1;
return len - 3 * K;
};
int ans = N + N;
for (int i = 0; i < N; i++) {
ans = min(ans, Solve(i));
}
if (ans > N) {
ans = -1;
}
cout << ans << "\n";
return 0;
}
