// File jjooii2.cpp created on 01.10.2025 at 09:00:10
#include <bits/stdc++.h>
using i64 = long long;
#ifdef DEBUG
#include "/home/ahmetalp/Desktop/Workplace/debug.h"
#else
#define debug(...) void(23)
#endif
int main() {
std::ios::sync_with_stdio(false);
std::cin.tie(nullptr);
int N, K;
std::cin >> N >> K;
std::string S;
std::cin >> S;
std::vector<int> A(N);
for (int i = 0; i < N; ++i) {
if (S[i] == 'J') {
A[i] = 0;
} else if (S[i] == 'O') {
A[i] = 1;
} else {
A[i] = 2;
}
}
std::vector<std::vector<int>> pre(N + 1, std::vector<int>(3));
for (int i = 0; i < N; ++i) {
pre[i + 1] = pre[i];
pre[i + 1][A[i]] += 1;
}
int ans = N + 1;
int p0 = 0, p1 = 0, p2 = 0;
for (int i = 0; i < N; ++i) {
if (S[i] != 'J') {
continue;
}
while (p0 != N + 1 && pre[p0][0] - pre[i][0] != K) {
p0++;
}
if (p0 == N + 1) {
break;
}
while (p1 != N + 1 && pre[p1][1] - pre[p0][1] != K) {
p1++;
}
if (p1 == N + 1) {
break;
}
while (p2 != N + 1 && pre[p2][2] - pre[p1][2] != K) {
p2++;
}
if (p2 == N + 1) {
break;
}
ans = std::min(ans, p2 - i);
}
if (ans == N + 1) {
std::cout << "-1\n";
} else {
std::cout << ans - 3 * K << '\n';
}
return 0;
}
