#include <bits/stdc++.h>
using namespace std;
int p[200001],q[200001];
int main()
{
int n,k;
string s;
cin>>n>>k>>s;
int cnt = 0;
for(int l=0,r=0;r<n;r++){
cnt+=s[r] == 'J';
while(cnt >= k){
if(cnt==k && s[l] == 'J')break;
cnt-=s[l] == 'J';
l++;
}
if(cnt == k)p[r] = r-l+1 - k;
else p[r] = -1;
}
cnt = 0;
for(int l=n-1,r=n-1;l>=0;l--){
cnt+=s[l] == 'I';
while(cnt >= k){
if(cnt==k && s[r] == 'I')break;
cnt-=s[r] == 'I';
r--;
}
if(cnt == k)q[l] = r-l+1 - k;
else q[l] = -1;
}
int ans = 1e9;
cnt = 0;
for(int l=0,r=0;r<n;r++){
cnt+=s[r] == 'O';
while(cnt >= k){
if(cnt==k && s[l] == 'O')break;
cnt-=s[l] == 'O';
l++;
}
if(l && p[l-1] != -1 && r+1<n && q[r+1] != -1 && cnt == k){
ans = min(ans,p[l-1] + q[r+1] + r-l+1-cnt);
}
}
if(ans == 1e9)ans = -1;
cout<<ans;
}
