#include<iostream>
#include<vector>
#include<set>
#include<algorithm>
using namespace std;
#define ll long long
#define mp make_pair
#define endl "\n"
#define out(x) cout<< #x << " = " << x << endl
int n,k,J,O,I;
string s;
vector<int> v[3];
signed main(){
cin>>n>>k>>s;
v[0].push_back(0); v[1].push_back(0); v[2].push_back(0);
for (int i=0;i<n;i++){
if (s[i] == 'J') v[0].push_back(i+1), J++;
if (s[i] == 'O') v[1].push_back(i+1), O++;
if (s[i] == 'I') v[2].push_back(i+1), I++;
}
int ans = 1e9;
for (int i=k;i<=J;i++){
int opos = lower_bound(v[1].begin(),v[1].end(),v[0][i]) - v[1].begin();
if (opos + k - 1 > O) continue;
int oend = v[1][opos + k - 1];
int ipos = lower_bound(v[2].begin(),v[2].end(),oend) - v[2].begin();
if (ipos + k - 1 > I) continue;
int iend = v[2][ipos + k - 1];
ans = min(ans, iend - v[0][i-k+1] + 1 - k*3);
}
cout<<(ans == 1e9 ? -1 : ans)<<endl;
}
