#include <cstdio>
#include <stdio.h>
#include <stdbool.h>
#include <iostream>
#include <map>
#include <vector>
#include <climits>
#include <stack>
#include <string>
#include <queue>
#include <algorithm>
#include <set>
#include <unordered_set>
#include <unordered_map>
#include <cmath>
#include <cctype>
#include <bitset>
#include <iomanip>
#include <cstring>
#include <numeric>
using namespace std;
#define int long long
#define pb push_back
#define mp make_pair
#define pii pair<int, int>
#define fi first
#define se second
int n, k, ans=LLONG_MAX/2;
vector<vector<int> > psum;
int bs(int a, int i){
int low=a-1, high=n+1;
while (low+1<high){
int mid=(low+high)/2;
if (psum[i][mid]-psum[i][a-1]>=k)high=mid;
else low=mid;
}
return high;
}
int32_t main(){
string s;
cin>>n>>k>>s;
psum.resize(3, vector<int>(n+1, 0));
for (int i=0; i<n; ++i){
for (int j=0; j<3; ++j)psum[j][i+1]+=psum[j][i];
++psum[(s[i]=='J'?0:(s[i]=='O'?1:2))][i+1];
}
for (int i=1; i<=n; ++i){
int id=bs(i, 0);
id=bs(id+1, 1), id=bs(id+1, 2);
if (id<=n)ans=min(ans, id-i+1-3*k);
}
cout<<(ans==LLONG_MAX/2?-1:ans);
}
