#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
using namespace std;
using namespace __gnu_pbds;
using ll = long long;
#define int ll
#define fore(a, b, c) for(int a=b; a<c; ++a)
#define vi vector<int>
#define pb push_back
#define ii pair<int,int>
#define F first
#define S second
#define all(x) x.begin(), x.end()
#define sz(x) (int) x.size()
#define endl '\n'
#define dbg(x) cerr << #x << ": " << x << endl
#define raya cerr << "========================" << endl
const int N = 2e5+5;
int n, k;
string s;
int contJ[N], contO[N], contI[N];
int findj(int i){
int low = i, high = n-1;
int j = -1;
while(low <= high){
int mid = low + (high - low) / 2;
if(contJ[mid] - (i == 0 ? 0 : contJ[i-1]) >= k){
j = mid;
high = mid-1;
} else {
low = mid+1;
}
}
return j;
}
int findl(int j){
int low = j+1, high = n-1;
int l = -1;
while(low <= high){
int mid = low + (high - low) / 2;
if(contO[mid] - contO[j] >= k){
l = mid;
high = mid-1;
} else {
low = mid+1;
}
}
return l;
}
int findm(int l){
int low = l+1, high = n-1;
int m = -1;
while(low <= high){
int mid = low + (high - low) / 2;
if(contI[mid] - contI[l] >= k){
m = mid;
high = mid-1;
} else {
low = mid+1;
}
}
return m;
}
signed main(){
ios::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cin >> n >> k;
cin >> s;
// contJ
for(int i=0; i<n; ++i){
contJ[i] += (s[i] == 'J');
contO[i] += (s[i] == 'O');
contI[i] += (s[i] == 'I');
contJ[i+1] = contJ[i];
contO[i+1] = contO[i];
contI[i+1] = contI[i];
}
int ans = n;
for(int i=0; i<n; ++i) if(s[i] == 'J'){
// then go to j
int j = findj(i);
if(j == -1) continue;
// then go to l
int l = findl(j);
if(l == -1) continue;
// then go to m
int m = findm(l);
if(m == -1) continue;
int cur = j - i + 1 - k;
cur += l - j - k;
cur += m - l - k;
ans = min(ans, cur);
}
// assert(ans > -1);
cout << (ans == n ? -1 : ans) << endl;
}
