// READ & UNDERSTAND
// ll, int overflow, array bounds, memset(0)
// special cases (n=1?), n+1 (1-index)
// do smth instead of nothing & stay organized
// WRITE STUFF DOWN
#include <bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define pb push_back
#define all(x) (x).begin(), (x).end()
#define MOD 1000000007
typedef long long ll;
typedef pair <int, int> ii;
typedef pair <ll, ll> pll;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef long double ld;
ll INF=LLONG_MAX;
int const mxn=2e5+5;
vi dp[3]; //JOI,pos
int main()
{
ios_base::sync_with_stdio(0); cin.tie(0);
//freopen("input.txt","r",stdin); freopen("output.txt","w",stdout);
int n,k; string s;
cin >> n >> k >> s;
dp[0].emplace_back(0);
dp[1].emplace_back(0);
dp[2].emplace_back(0);
for(int i=0; i<n; i++){
dp[0].emplace_back(dp[0].back()+(s[i]=='J'?1:0));
dp[1].emplace_back(dp[1].back()+(s[i]=='O'?1:0));
dp[2].emplace_back(dp[2].back()+(s[i]=='I'?1:0));
}
int ans=1e9;
int JJ = dp[0][n], OO = dp[1][n], II = dp[2][n];
for(int j=k; j<=JJ; j++){
int posj = lower_bound(all(dp[0]),j)-dp[0].begin();
int cnto = dp[1][posj];
for(int o=cnto+k; o<=OO; o++){
int poso = lower_bound(all(dp[1]),o)-dp[1].begin();
int cnti = dp[2][poso];
for(int i=cnti+k; i<=II; i++){
int posi = lower_bound(all(dp[2]),i)-dp[2].begin();
int st = dp[0][posj]-k+1;
int posini = lower_bound(all(dp[0]),st)-dp[0].begin();
int len = posi-posini+1;
int cnt = len-k*3;
ans = min(ans,cnt);
}
}
}
if(ans==1e9)ans=-1;
cout << ans << endl;
}
