#pragma GCC optimize("O3,inline")
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
using namespace std;
using ll = long long;
#define int ll
using ii = pair<int, int>;
using iii = pair<ii, int>;
constexpr int INF = 1e18 + 5;
constexpr int MAXN = 200'000 + 5;
constexpr int MOD = 1e9 + 7;
signed main() {
ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);
int N, K; cin >> N >> K;
string S; cin >> S;
vector<int> jindices;
vector<int> oindices;
vector<int> iindices;
for (int i = 0; i < N; ++i) {
if (S[i] == 'J') {
jindices.emplace_back(i);
}
else if (S[i] == 'O') oindices.emplace_back(i);
else iindices.emplace_back(i);
}
vector<int> dpJ(N, INF);
for (int i = K-1; i < (int)jindices.size(); ++i) {
int last = jindices[i-(K-1)];
int curr = jindices[i];
int rem = (curr - last + 1) - K;
dpJ[curr] = rem;
}
int ptr = -1;
vector<int> dpO(N, INF);
int opt = INF;
for (int i = K-1; i < (int)oindices.size(); ++i) {
int last = oindices[i-(K-1)];
int curr = oindices[i];
while (ptr + 1 < (int)jindices.size() && jindices[ptr + 1] < last) {
ptr++;
opt = min(opt, dpJ[jindices[ptr]] - (jindices[ptr] + 1));
}
dpO[curr] = min(INF, (curr - last + 1) - K + last-1 + opt + 1);
}
ptr = -1;
opt = INF;
int ans = INF;
for (int i = K-1; i < (int)iindices.size(); ++i) {
int last = iindices[i-(K-1)];
int curr = iindices[i];
while (ptr + 1 < (int)oindices.size() && oindices[ptr + 1] < last) {
ptr++;
opt = min(opt, dpO[oindices[ptr]] - (oindices[ptr] + 1));
}
ans = min(ans, (curr - last + 1) - K + last-1 + opt + 1);
}
cout << (ans == INF ? -1ll : ans);
}
