#define NDEBUG
#ifdef NDEBUG
#define dbg(TXTMSG) if constexpr (false) cerr << "lol"
#define dbgv(VARN) ((void)0)
#define dbgfor(COND) if constexpr (false) for (COND)
#else
#define _GLIBCXX_DEBUG 1
#define _GLIBCXX_DEBUG_PEDANTIC 1
#pragma GCC optimize("trapv")
#define dbg(TXTMSG) cerr << "\n" << TXTMSG
#define dbgv(VARN) cerr << "\n" << #VARN << " = "<< VARN << ", line " << __LINE__ << "\n"
#define dbgfor(COND) for (COND)
#endif
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pll = pair<ll,ll>;
#define e0 first
#define e1 second
constexpr ll INFTY = 2e18;
int main()
{
ll N,K;
cin >> N >> K;
string S;
cin >> S;
vector<ll> Js;
vector<ll> Os;
vector<ll> Is;
for (ll i = 0; i < N; ++i)
{
if (S[i]=='J')
{
Js.push_back(i);
}
else if (S[i]=='O')
{
Os.push_back(i);
}
else
{
assert(S[i]=='I');
Is.push_back(i);
}
}
ll outp=INFTY;
for (ll i = 0; i+K <= ll(Js.size()); ++i)
{
ll jidx = i+K-1;
ll oidx = lower_bound(Os.cbegin(),Os.cend(), Js[jidx])-Os.cbegin()+K-1;
if (ll(Os.size())<=oidx) break;
ll iidx = lower_bound(Is.cbegin(), Is.cend(), Os[oidx])-Is.cbegin()+K-1;
if (ll(Is.size())<=iidx) break;
outp = min(outp,Is[iidx]-Js[i]+1);
}
cout << ((outp==INFTY)?-1:(outp-3*K)) << "\n";
}
