#include<bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("avx,avx2,fma")
using namespace __gnu_pbds;
#define ordered_set tree<int, null_type,less_equal<int>, rb_tree_tag,tree_order_statistics_node_update>
#define ll long long
#define MOD 998244353
#define MAXN 2e5
#define SIZE 314
using namespace std;
int solve(string &s, int start, int n, int k, vector<ll> &J, vector<ll> &O, vector<ll> &I){
int curr = start;
int l = curr, r = n - 1, mid;
int bestPos = -1;
while(l <= r){
mid = (r - l) / 2 + l;
ll count = J[mid + 1] - J[curr];
if (count >= k){
bestPos = mid;
r = mid - 1;
}
else{
l = mid + 1;
}
}
if (bestPos == -1) return -1;
curr = bestPos + 1;
l = curr, r = n - 1;
bestPos = -1;
while(l <= r){
mid = (r - l) / 2 + l;
ll count = O[mid + 1] - O[curr];
if (count >= k){
bestPos = mid;
r = mid - 1;
}
else{
l = mid + 1;
}
}
if (bestPos == -1) return -1;
curr = bestPos + 1;
l = curr, r = n - 1;
bestPos = -1;
while(l <= r){
mid = (r - l) / 2 + l;
ll count = I[mid + 1] - I[curr];
if (count >= k){
bestPos = mid;
r = mid - 1;
}
else{
l = mid + 1;
}
}
if (bestPos == -1) return -1;
curr = bestPos + 1;
return curr - start;
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int n, k;
cin >> n >> k;
string s;
cin >> s;
vector<ll> J(n + 1), O(n + 1), I(n + 1);
J[0] = O[0] = I[0] = 0;
for (int i = 0; i < n; i++){
J[i + 1] = J[i];
O[i + 1] = O[i];
I[i + 1] = I[i];
if (s[i] == 'J') J[i+1]++;
else if (s[i] == 'O') O[i+1]++;
else I[i+1]++;
}
if (solve(s, 0, n, k, J, O, I) == -1){
cout << "-1\n";
return 0;
}
ll ans = n;
for (int i = 0; i < n; i++){
ll curr = solve(s, i, n, k, J, O, I);
if (curr != -1 && curr < ans) ans = curr;
}
cout << ans - 3 * k << "\n";
return 0;
}
