#ifdef __AVX2__
#pragma GCC target "avx2"
#endif
#pragma GCC optimize "O3"
#pragma GCC optimize "unroll-loops"
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
using namespace std;
#define int long long
#define elif else if
#define all(l) begin(l),end(l)
#define rall(l) rbegin(l),rend(l)
#define append push_back
#define print(l) for(auto i:l) cout<<i<<' '; cout<<endl;
#define pprint(a,b) cout<<a<<' '<<b<<endl;
#define inp(l) for(auto &i:l) cin>>i;
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
#define pai make_pair
#define endl "\n"
#define pii pair<int,int>
#define fi first
#define se second
#define vec vector
// const int mod=998244353;
const int mod1=998244353;
const int mod=1e9+7;
const int N=2e5+5;
int iter=1,itera=1;
void solve(){
int n,k;
cin>>n>>k;
string s;
cin>>s;
s='.'+s;
vec<int>J(n+1,-1e18),O(n+1,0),I(n+1,0);
deque<int>j1,o1,i1;
for(int i=1;i<=n;i++){
if(s[i]=='J'){
j1.append(i);
if(j1.size()>k) j1.pop_front();
}
elif(s[i]=='O'){
o1.append(i);
if(o1.size()>k) o1.pop_front();
}
else{
i1.append(i);
if(i1.size()>k) i1.pop_front();
}
if(j1.size()==k) J[i]=j1[0];
if(o1.size()==k) O[i]=o1[0];
if(i1.size()==k) I[i]=i1[0];
}
int ans=1e18;
for(int i=1;i<=n;i++){
int l=J[O[I[i]]];
ans=min(ans,(i-l+1)-3*k);
}
if(ans>n) cout<<"-1\n";
else cout<<ans<<endl;
}
signed main(){
// freopen("","r",stdin);
// freopen("","w",stdout);
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
cout<<fixed<<setprecision(20);
// cin>>itera;
for(iter=1;iter<=itera;iter++) solve();
}
