#include <bits/stdc++.h>
using namespace std;
const int N = 2e5 + 12, MOD = 1e9 + 7;
typedef long long ll;
int n,k,A[N],B[N],C[N];
string s;
vector<int> a,b,c;
void test(){
cin >> n >> k >> s;
s = "#" + s;
for(int i = 1;i <= n;i++){
A[i] = A[i - 1];
B[i] = B[i - 1];
C[i] = C[i - 1];
if(s[i] == 'J'){
A[i]++;
a.push_back(i);
}else if(s[i] == 'O'){
B[i]++;
b.push_back(i);
}else{
C[i]++;
c.push_back(i);
}
}
if(min({(int)a.size(),(int)b.size(),(int)c.size()}) < k){
cout << -1 << '\n';
return;
}
int res = 1e9;
for(int i = 0;i < (int)a.size();i++){
if(i + k - 1 >= (int)a.size()) break;
int l = -1,r = c.size() - k + 1;
while(r - l > 1){
int j = (l + r) >> 1;
if(B[c[j]] - B[a[i + k - 1]] >= k){
res = min(res,n - k * 3 - (a[i] - 1) - (n - c[j + k - 1]));
r = j;
}else{
l = j;
}
}
for(int j = (int)c.size() - k;j >= 0;j--){
if(B[c[j]] - B[a[i + k - 1]] >= k){
// res = min(res,n - k * 3 - (a[i] - 1) - (n - c[j + k - 1]));
}else break;
}
}
if(res == 1e9) res = -1;
cout<< res << '\n';
}
int main()
{
ios_base::sync_with_stdio(0);cin.tie(0);
int T = 1;
// cin >> T;
while (T--)
test();
}
