// #pragma GCC optimize("O3")
// #pragma GCC optimization("Ofast,unroll-loops")
// #pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
#include <bits/stdc++.h>
using namespace std;
#define int long long
#define ll long long
#define FOR(i, l, r) for (int i = (l); i <= (r); i++)
#define FOD(i, r, l) for (int i = (r); i >= (l); i--)
#define fi first
#define se second
#define pii pair<int, int>
const ll mod = 1e9 + 7;
const int MAXN = 2e5 + 5;
const ll oo = 1e18+7;
const int base = 0;
int n, k;
string s;
int nxt1[MAXN], nxt2[MAXN], nxt3[MAXN];
signed main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
// freopen("test.txt", "r", stdin);
// freopen("o2.out", "w", stdout);
// if(fopen(".inp", "r")){
// freopen(".inp", "r", stdin);
// freopen(".out", "w", stdout);
// }
cin >> n >> k;
cin >> s;
s="."+s;
s=s+".";
int dm, j;
dm=0, j=n+1;
FOD(i, n, 1){
if(s[i]=='J'){
dm++;
}
if(dm>=k){
while(dm>=k){
if(s[j]=='J'){
dm--;
}
j--;
}
if(s[j+1]=='J'){
j++;
dm++;
}
}
if(dm==k){
nxt1[i]=j;
}
}
dm=0, j=n+1;
FOD(i, n, 1){
if(s[i]=='O'){
dm++;
}
if(dm>=k){
while(dm>=k){
if(s[j]=='O'){
dm--;
}
j--;
}
if(s[j+1]=='O'){
j++;
dm++;
}
}
if(dm==k){
nxt2[i]=j;
}
}
dm=0, j=n+1;
FOD(i, n, 1){
if(s[i]=='I'){
dm++;
}
if(dm>=k){
while(dm>=k){
if(s[j]=='I'){
dm--;
}
j--;
}
if(s[j+1]=='I'){
j++;
dm++;
}
}
if(dm==k){
nxt3[i]=j;
}
}
int ans=oo;
FOR(i, 1, n){
int a=nxt1[i];
if(a==0) break;
a=nxt2[a+1];
if(a==0) break;
a=nxt3[a+1];
if(a==0) break;
ans=min(ans, a-i+1-k*3);
}
if(ans==oo) ans=-1;
cout << ans;
return 0;
}
