#include <bits/stdc++.h>
using namespace std;
const int MM = 2e5+5;
int n,k,psa[3][MM]; string s;
int f(char c){
return (c == 'J' ? 0 : c == 'O' ? 1 : 2);
}
int next_k(int i, int j){
if(i > n) return -1;
int lo = i, hi = n;
while(lo < hi){
int mid = (lo+hi)/2;
if(psa[j][mid] - psa[j][i-1] >= k) hi = mid;
else lo = mid+1;
}
if(psa[j][lo] - psa[j][i-1] < k) return -1;
return lo;
}
int main(){
cin >> n >> k >> s;
s = ' ' + s;
for(int i = 1; i <= n; i++){
psa[0][i] += psa[0][i-1];
psa[1][i] += psa[1][i-1];
psa[2][i] += psa[2][i-1];
psa[f(s[i])][i] += 1;
}
int ans = 1e9;
for(int i = 1; i <= n; i++){
int v = next_k(i,0),len = 0,p = i;
if(v == -1) continue;
len += v - p+1;
p = v+1;
v = next_k(p,1);
if(v == -1) continue;
len += v - p+1;
p = v+1;
v = next_k(p,2);
if(v == -1) continue;
len += v - p + 1;
ans = min(ans,len - 3*k);
}
cout << (ans==1e9?-1:ans) << "\n";
}
