#include <bits/stdc++.h>
typedef long long ll;
#define pb push_back
#define MS(x,y) memset((x),(y),sizeof((x)))
#define MN 1000000007
using namespace std;
int main()
{
#if LOCAL_DEBUG
fstream cin("in.txt");
#endif
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int n,k;
cin>>n>>k;
int pref[3][n+5];
MS(pref,0);
/// J pa O pa I
string s;
cin>>s;
for (int i = 0; i<n; i++) {
pref[0][i+1] = pref[0][i] + (s[i] == 'J');
pref[1][i+1] = pref[1][i] + (s[i] == 'O');
pref[2][i+1] = pref[2][i] + (s[i] == 'I');
}
int curr[3];
int res = INT_MAX/2;
for (int i = 0; i<=n; i++) {
for (int j = 0; j<3; j++) curr[j] = pref[j][i];
int newPos = i;
int resC = 0;
for (int j = 0; j<3; j++) {
auto itJ = lower_bound(pref[j]+newPos,pref[j]+(n+1),curr[j]+k);
if (itJ == pref[j]+(n+1)) {resC = INT_MAX/2; break; }
newPos = itJ-pref[j];
for (int k = 0; k<3; k++) if (k != j) resC += pref[k][newPos]-curr[k];
for (int k = 0; k<3; k++) curr[k] = pref[k][newPos];
}
res = min(res,resC);
}
if (res != INT_MAX/2) cout<<res<<endl;
else cout<<-1<<endl;
return 0;
}
