#include<bits/stdc++.h>
using namespace std;
#define int long long
#define ld long double
const int N = 2e5 + 5, mod = 1e9+7;
vector<pair<int, int> > vec;
multiset<int> st;
int minn = 1e9;
int last[N];
int n;
int pref[3][N], k;
int findd(int ind, int type){
int l = ind; int r = n; int anss = -1;
while(l <= r){
int mid = l + (r-l)/2;
if( pref[type][mid] - pref[type][ind - 1] >= k){
anss = mid;
r = mid - 1;
}else l = mid + 1;
}
return anss;
}
int32_t main(){
cin >> n >> k;
string s; cin >> s;
s = "*" + s;
for(int i = 1; i <= n; i++){
pref[0][i] = pref[0][i - 1] + (s[i] == 'J');
pref[1][i] = pref[1][i - 1] + (s[i] == 'O');
pref[2][i] = pref[2][i - 1] + (s[i] == 'I');
}
for(int i = 1; i <= n; i++){
if(s[i] != 'J') continue;
int mekJ = findd(i, 0);
if (mekJ == -1) break;
int mekO = findd(mekJ, 1);
if (mekO == -1) break;
int mekI = findd(mekO, 2);
if (mekI == -1) break;
int l = i;
int r = mekI;
minn = min(minn, r - l + 1 - 3 * k);
}
if(minn == 1e9) cout << -1 << endl;
else
cout << minn << endl;
}
