#include<bits/stdc++.h>
using namespace std;
/// ----------------------------- (Debug) ------------------------------
#define sim template < class c
#define ris return * this
#define dor > debug & operator <<
#define eni(x) sim > typename enable_if<sizeof dud<c>(0) x 1, debug&>::type operator<<(c i) {
sim > struct rge { c b, e; }; sim > rge<c> range(c i, c j) { return rge<c>{i, j}; }
sim > auto dud(c* x) -> decltype(cerr << *x, 0); sim > char dud(...);
struct debug {
#ifndef ONLINE_JUDGE
eni(!=) cerr << boolalpha << i; ris; }
eni(==) ris << range(begin(i), end(i));}
sim, class b dor(pair < b, c > d) {ris << "(" << d.first << ", " << d.second << ")";}
sim dor(rge<c> d) {*this << "["; for (auto it = d.b; it != d.e; ++it) *this << ", " + 2 * (it == d.b) << *it; ris << "]";}
#else
sim dor(const c&) { ris; }
#endif
};
vector<char*> tokenizer(const char* args) {
char *token = new char[111]; strcpy(token, args); token = strtok(token, ", ");
vector<char*> v({token});
while(token = strtok(NULL,", ")) v.push_back(token);
return reverse(v.begin(), v.end()), v;
}
void debugg(vector<char*> args) { cerr << "\b\b "; }
template <typename Head, typename... Tail>
void debugg(vector<char*> args, Head H, Tail... T) {
debug() << " [" << args.back() << ": " << H << "] ";
args.pop_back(); debugg(args, T...);
}
#define harg(...) #__VA_ARGS__
#ifndef ONLINE_JUDGE
#define dbg(...) { debugg(tokenizer(harg(__VA_ARGS__, \b\t-->Line)), __VA_ARGS__, __LINE__); cerr << endl;}
#else
#define dbg(...) { }
#endif
/// -----------------------------------------------------------------------
typedef long long ll;
typedef vector<int> vi;
typedef pair<int,int> pi;
typedef vector<pi> vpi;
#define IOS ios::sync_with_stdio(0); cin.tie(0); cout.tie(0)
#define F first
#define S second
#define PB push_back
#define EB emplace_back
#define MP make_pair
#define print(x) cout << x << "\n";
#define REP(i,a,b) for(i=a;i<=b;i++)
#define RAP(i,a,b) for(i=a;i>=b;i--)
#define spa <<" "<<
#define all(x) (x).begin(), (x).end()
#define sz(x) (int)x.size()
ll mod=1e9+7;
const int MX=0x3f3f3f3f;
int main(){
IOS;
int n,k,i;
cin>>n>>k;
string s;
cin>>s;
vi J,O,I;
REP(i,0,n-1){
if(s[i]=='J') J.EB(i);
else if(s[i]=='O') O.EB(i);
else I.EB(i);
}
// dbg(J,O,I);
int ans=MX,j;
REP(i,0,sz(J)-k){
j=upper_bound(all(O),J[i+k-1])-O.begin();
if(j+k-1>=sz(O)) break;
j=upper_bound(all(I),O[j+k-1])-I.begin();
if(j+k-1>=sz(I)) break;
// dbg(i,j,J[i],I[j+k-1]);
ans=min(ans,I[j+k-1]-J[i]+1-3*k);
}
if(ans==MX) cout<<-1;
else cout<<ans;
return 0;
}
