#include <bits/stdc++.h>
using namespace std;
const long long INFll = (long long) 1e18 + 10;
const int INFii = (int) 1e9 + 10;
typedef long long ll;
typedef int ii;
typedef long double dbl;
#define endl '\n'
#define sc second
#define fr first
#define mp make_pair
#define pb push_back
#define all(x) x.begin(), x.end()
#define maxn 220000
ii n, k, a[maxn], dp[maxn][5], pos[5][maxn], px[maxn], qtd[5];
string s;
ii sol(ii i, ii p) {
if(p == 3) return 0;
if(i == n+1) return INFii;
if(dp[i][p] != -1) return dp[i][p];
ii ans = INFii;
ans = min(ans, sol(i+1,p)+1);
if(p == 0) ans = min(ans, sol(i+1,p));
if(a[i] == p && px[i]+k-1 <= qtd[p]) {
ans = min(ans, sol(pos[p][px[i]+k-1] + 1 ,p+1) + pos[p][px[i]+k-1]-i+1-k);
}
return dp[i][p] = ans;
}
int main() {
ios::sync_with_stdio(false); cin.tie(0);
//freopen("in.in", "r", stdin);
//freopen("out.out", "w", stdout);
cin >> n >> k >> s;
for(ii i = 1; i <= n; i++) {
dp[i][0] = dp[i][1] = dp[i][2] = dp[i][3] = -1;
if(s[i-1] == 'J') {
a[i] = 0;
px[i] = ++qtd[0];
pos[0][qtd[0]] = i;
}
else if(s[i-1] == 'O') {
a[i] = 1;
px[i] = ++qtd[1];
pos[1][qtd[1]] = i;
}
else {
a[i] = 2;
px[i] = ++qtd[2];
pos[2][qtd[2]] = i;
}
}
if(sol(1,0) >= INFii) cout << -1 << endl;
else cout << sol(1,0) << endl;
}
