#include <bits/stdc++.h>
#define speed ios_base::sync_with_stdio(0); cin.tie(0)
#define all(x) (x).begin(),(x).end()
#define F first
#define S second
using namespace std;
const int INF=0x3f3f3f3f;
int main() {
speed;
int n,k;
string s;
cin>>n>>k;
cin>>s;
s='-'+s;
vector<int> dp(n+1);
int l=1;
int cnt=0;
dp[0]=INF;
for (int i=1;i<=n;i++) {
cnt+=(s[i]=='J');
while (cnt-(s[l]=='J')>=k) {
cnt-=(s[l]=='J');
l++;
}
if (cnt==k) dp[i]=i-l+1-k;
else dp[i]=INF;
// cout<<dp[i]<<" ";
}
// cout<<"\n";
l=1;
cnt=0;
vector<int> dp2(n+1);
dp2[0]=INF;
for (int i=1;i<=n;i++) {
cnt+=(s[i]=='O');
while (cnt-(s[l]=='O')>=k) {
cnt-=(s[l]=='O');
l++;
}
if (cnt==k and dp[l-1]!=INF) dp2[i]=i-l+1-k+dp[l-1];
else dp2[i]=INF;
// cout<<dp2[i]<<" ";
}
// cout<<"\n";
l=1;
cnt=0;
int ans=INF;
for (int i=1;i<=n;i++) {
cnt+=(s[i]=='I');
while (cnt-(s[l]=='I')>=k) {
cnt-=(s[l]=='I');
l++;
}
if (cnt==k and dp2[l-1]!=INF) ans=min(ans,i-l+1-k+dp2[l-1]);
// cout<<ans<<" ";
}
cout<<(ans==INF?-1:ans)<<"\n";
return 0;
}
