#include <bits/stdc++.h>
using namespace std;
#define all(x) x.begin(), x.end()
#define pb push_back
#define ln '\n'
#define int long long
template <class _T>
bool chmin(_T &x, const _T &y){
bool flag = false;
if ( x > y ){
x = y; flag |= true;
}
return flag;
}
template <class _T>
bool chmax(_T &x, const _T &y){
bool flag = false;
if ( x < y ){
x = y; flag |= true;
}
return flag;
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int n, k; cin >> n >> k;
string s; cin >> s;
const int inf = 1e15 + 1;
vector <int> pf(n + 1, inf), sf(n + 1, inf);
deque <int> dq;
for ( int i = 0; i < n; i++ ){
if ( s[i] == 'J' ){
dq.pb(i);
}
if ( (int)dq.size() > k ){
dq.pop_front();
}
if ( (int)dq.size() < k ){
continue;
}
pf[i + 1] = (i - dq[0] + 1) - k;
}
dq.clear();
for ( int i = n - 1; i >= 0; i-- ){
if ( s[i] == 'I' ){
dq.push_front(i);
}
if ( (int)dq.size() > k ){
dq.pop_back();
}
if ( (int)dq.size() < k ){
continue;
}
sf[i + 1] = (dq[k - 1] - i + 1) - k;
}
vector <int> dp(n + 2, inf);
for ( int i = n; i > 0; i-- ){
dp[i] = min(dp[i + 1], sf[i] + i);
}
int it = 1, f = 0, ans = inf;
for ( int i = 1; i <= n; i++ ){
if ( i > 1 ){
f -= (s[i - 1] == 'O');
}
while ( it + 1 <= n and f < k ){
f += (s[it] == 'O');
it++;
}
if ( f < k ){
break;
}
chmin(ans, (pf[i] - i) + dp[it + 1] - k - 1);
}
if ( ans > n ){
ans = -1;
}
cout << ans;
cout << '\n';
}
