#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <bits/stdc++.h>
using namespace std;
using namespace __gnu_pbds;
typedef long long ll;
typedef long double ldb;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef pair<ldb,ldb> pdd;
#define ff(i,a,b) for(int i = a; i <= b; i++)
#define fb(i,b,a) for(int i = b; i >= a; i--)
#define trav(a,x) for (auto& a : x)
#define sz(a) (int)(a).size()
#define pb push_back
#define fi first
#define se second
#define lb lower_bound
#define ub upper_bound
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
template<typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
// os.order_of_key(k) the number of elements in the os less than k
// *os.find_by_order(k) print the k-th smallest number in os(0-based)
const int mod = 1000000007;
const int inf = 1e9 + 5;
const int mxN = 200005;
int n, k;
string s;
queue<int> J, O, I;
int pref[mxN];
int sufi[mxN];
int f(char c){
if(c == 'J')return 0;
if(c == 'O')return 1;
if(c == 'I')return 2;
assert(false);
}
int main() {
cin.tie(0)->sync_with_stdio(0);
cin >> n >> k >> s; s = ' ' + s;
pref[0] = inf;
ff(i,1,n){
if(f(s[i]) == 0)J.push(i);
if(sz(J) > k)J.pop();
pref[i] = pref[i - 1];
if(sz(J) >= k){
int l = J.front();
int r = i;
pref[i] = (r - l + 1) - k;
}
}
sufi[n + 1] = inf;
fb(i,n,1){
if(f(s[i]) == 2)I.push(i);
if(sz(I) > k)I.pop();
sufi[i] = sufi[i + 1];
if(sz(I) >= k){
int l = i;
int r = I.front();
sufi[i] = (r - l + 1) - k;
}
}
int rez = inf; // O
ff(i,1,n){
if(f(s[i]) == 1)O.push(i);
if(sz(O) > k)O.pop();
if(sz(O) >= k){
int l = O.front();
int r = i;
rez = min(rez, pref[l - 1] + sufi[r + 1] + (r - l + 1) - k);
}
}
cout << (rez == inf ? -1 : rez) << '\n';
return 0;
}
/*
10 2
OJIJOIOIIJ
9 3
JJJOOOIII
9 1
IIIOOOJJJ
// probati bojenje sahovski
*/
