#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
//#include<bits/extc++.h>
//__gnu_pbds
int main(){
ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0);
int n,k;cin>>n>>k;
vector<int> J(n+1,0);
vector<int> O(n+1,0);
vector<int> I(n+1,0);
string s;cin>>s;
for(int i=1;i<=n;i++){
J[i]=J[i-1];
O[i]=O[i-1];
I[i]=I[i-1];
if(s[i-1]=='J') J[i]++;
if(s[i-1]=='O') O[i]++;
if(s[i-1]=='I') I[i]++;
}
int ans = 1e9;
for(int i=1;i<=n;i++){
if(s[i-1]!='I') continue;
int l = -1;int r = i;
int t = i;
while(r-l>1){
int mid = (l+r)/2;
if(I[t]-I[mid]>=k) l = mid;
else r = mid;
}
r = l;l = -1;
if(r<=0) continue;
t = r;
while(r-l>1){
int mid = (l+r)/2;
if(O[t]-O[mid]>=k) l = mid;
else r = mid;
}
r = l;l = -1;
if(r<=0) continue;
t = r;
while(r-l>1){
int mid = (l+r)/2;
if(J[t]-J[mid]>=k) l=mid;
else r = mid;
}
if(l>=0){
ans = min(ans,i-l-3*k);
}
}
if(ans>=1e7) cout<<-1<<"\n";
else cout<<ans<<"\n";
return 0;
}
