#include <bits/stdc++.h>
using namespace std;
#define int long long
#define double long double
#define FOR(i, l, r, d) for(int i=(l); i<=(r); i+=(d))
#define szof(x) ((int)(x).size())
#define vi vector<int>
#define pii pair<int, int>
#define F first
#define S second
#define pb push_back
#define eb emplace_back
#define mkp make_pair
const int INF = INT_MAX;
const int LNF = INF*INF;
const int MOD = 1000000007;
const double eps = 1e-12;
// #pragma GCC optimize("Ofast")
// #pragma GCC optimize("unroll-loops")
const int MAX = 200010;
int n, k;
int pref[4][MAX]; // J = 1, O = 2, I = 3
int jl, jr, il, ir;
int res = INF;
inline int query(int ql, int qr, char qc){
if(qc == 'J') return pref[1][qr] - pref[1][ql-1];
if(qc == 'O') return pref[2][qr] - pref[2][ql-1];
return pref[3][qr] - pref[3][ql-1];
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
cin>>n>>k;
FOR(i, 1, n, 1){
char owo;
cin>>owo;
FOR(j, 1, 3, 1) pref[j][i] = pref[j][i-1];
if(owo == 'J') pref[1][i]++;
if(owo == 'O') pref[2][i]++;
if(owo == 'I') pref[3][i]++;
}
FOR(j, 1, 3, 1) pref[j][n+1] = INF;
//cerr<<"ok"<<endl;
jl = 1, jr = 0, il = 0, ir = 0;
while(jl <= n){
// move [jl, jr]
while(jr <= n and query(jl, jr, 'J') < k) jr++;
il = max(il, jr + 1);
while(ir <= n and query(il, ir, 'I') < k) ir++;
if(ir > n) break;
//cerr<<"? "<<'['<<jl<<", "<<jr<<"] ["<<il<<", "<<ir<<"] "<<endl;
// move [il, ir] until [jr+1, ir-1] is ok
while(query(jr+1, il-1, 'O') < k){
il++;
while(ir <= n and query(il, ir, 'I') < k) ir++;
if(ir > n) break;
//cerr<<". "<<'['<<jl<<", "<<jr<<"] ["<<il<<", "<<ir<<"] "<<endl;
}
if(ir > n) break;
// update res
res = min(res, ir - jl + 1 - 3*k);
//cerr<<"! "<<'['<<jl<<", "<<jr<<"] ["<<il<<", "<<ir<<"] "<<endl;
jl++;
}
if(res < INF) cout<<res<<'\n';
else cout<<-1<<'\n';
return 0;
}
