#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const ll INFL = 1e18;
const int MN = 200100;
int pr[3][MN];
int main() {
ios::sync_with_stdio(0);cin.tie(0);
ll n,k;
cin >> n >> k;
string s;
cin >> s;
pr[0][0] = pr[0][1] = pr[0][2] = 0;
for(int i=0;i<n;i++) {
for(int j=0;j<3;j++) {
pr[j][i+1] = pr[j][i];
}
if(s[i] == 'J') {
pr[0][i+1]++;
} else if(s[i] == 'O') {
pr[1][i+1]++;
} else {
pr[2][i+1]++;
}
}
ll res = -1;
for(int i=0;i<n;i++) {
int u = i;
for(int j=0;j<3;j++) {
u = lower_bound(pr[j]+u,pr[j]+n+1,pr[j][u]+k)-pr[j];
}
if(u <= n) {
res = max(res,i+n-u);
}
}
if(res == -1) {
cout << -1 << '\n';
} else {
ll ans = n-3*k-res;
cout << ans << '\n';
}
}
