// #pragma optimize ("g",on)
// #pragma GCC optimize ("inline")
// #pragma GCC optimize ("Ofast")
// #pragma GCC optimize ("unroll-loops")
// #pragma GCC optimize ("03")
#include <bits/stdc++.h>
#define pb push_back
#define SS ios_base::sync_with_stdio(0);cin.tie(nullptr);cout.tie(nullptr);
#define int long long
#define all(v) v.begin(),v.end()
using namespace std;
mt19937 rnd(chrono::steady_clock::now().time_since_epoch().count());
const int N = 5e5 + 1, inf = 1e9, mod = 998244353;
void solve(){
int n, k;
cin >> n >> k;
string s;
cin >> s;
s = '!' + s;
vector<int> p[3];
for(int i = 1; i <= n; i++){
if(s[i] == 'J') p[0].push_back(i);
if(s[i] == 'O') p[1].push_back(i);
if(s[i] == 'I') p[2].push_back(i);
}
int dp[n + 2] = {}, dp1[n + 2] = {};
if(p[0].size() < k || p[1].size() < k || p[2].size() < k){
cout << -1;
return;
}
for(int i = 0; i <= n + 1; i++) dp[i] = dp1[i] = inf;
for(int i = 0; i + k - 1 < p[0].size(); i++){
int l = p[0][i], r = p[0][i + k - 1];
dp[r] = (r - l + 1) - k;
}
for(int i = p[2].size() - 1; i - k + 1 >= 0; i--){
int r = p[2][i], l = p[2][i - k + 1];
dp1[l] = (r - l + 1) - k;
}
int ans = inf;
for(int i = 1; i <= n; i++){
dp[i] = min(dp[i], dp[i - 1] + 1);
}
for(int i = n; i >= 1; i--){
dp1[i] = min(dp1[i], dp1[i + 1]);
}
for(int i = 0; i + k - 1 < p[1].size(); i++){
int l = p[1][i], r = p[1][i + k - 1];
ans = min(ans, dp[l - 1] + dp1[r + 1] + (r - l + 1) - k);
}
if(ans >= inf){
cout << -1;
return;
}
cout << ans;
}
signed main(){
SS
// freopen("trains.in", "r", stdin);
// freopen("trains.out", "w", stdout);
int t = 1;
// cin >> t;
while(t--){
solve();
}
}
