#include <bits/stdc++.h>
using namespace std;
#define int long long
#define vt vector
#define pb push_back
#define pob pop_back
#define endl '\n'
#define fi first
#define se second
signed main() {
int n , k;
cin >> n >> k;
string s;
cin >> s;
vt<int> J(n , 0);
vt<int> O(n , 0);
vt<int> I(n , 0);
vt<int> j;
for(int i=0 ; i<n ; i++){
if(s[i] == 'J')
j.pb(i);
}
J[0] += (s[0] == 'J');
O[0] += (s[0] == 'O');
I[0] += (s[0] == 'I');
for(int i=1 ; i<n ; i++){
J[i] = J[i-1] + (s[i] == 'J');
O[i] = O[i-1] + (s[i] == 'O');
I[i] = I[i-1] + (s[i] == 'I');
}
int ans = LLONG_MAX;
for(int i=0 ; i<j.size() ; i++){
int xx = k;
int yy;
int zz;
int e;
if(i !=0)
xx += J[i-1];
if(lower_bound(J.begin() , J.end() , xx) != J.end())
yy = lower_bound(J.begin() , J.end() , xx) - J.begin();
else continue;
yy = O[yy] + k;
if(lower_bound(O.begin() , O.end() , yy) != O.end())
zz = lower_bound(O.begin() , O.end() , yy) - O.begin();
else continue;
zz = I[zz] + k;
if(lower_bound(I.begin() , I.end() , zz) != I.end())
e = lower_bound(I.begin() , I.end() , zz) - I.begin();
else continue;
int end = e - i + 1;
end -= 3*k;
ans = min(ans , end);
}
if(ans == LLONG_MAX)
cout << -1 << endl;
else
cout << ans << endl;
}
