#include<iostream>
#include<vector>
#include<queue>
#include<stack>
#include<algorithm>
#include<limits.h>
#include<math.h>
#include<map>
#include<set>
#include<unordered_map>
#include<unordered_set>
#include<iomanip>
#include<cstring>
#include<random>
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
using namespace std;
int n,k;
string str;
int dpj[200001];
int dpo[200001];
int dpi[200001];
void fill(char c,int*dp){
dp[n]=n;
int r=0;
int cnt=k;
while(r<n){
if(str[r]==c)
cnt--;
if(cnt==0)
break;
r++;
}
for(int l=0;l<n;l++){
if(l&&str[l-1]==c&&r!=n){
r++;
while(r<n&&str[r]!=c)
r++;
}
dp[l]=r;
}
}
void solve(){
cin>>n>>k>>str;
fill('J',dpj);
fill('O',dpo);
fill('I',dpi);
/*for(int i=0;i<n;i++)
cout<<dpj[i]<<" ";
cout<<"\n";
for(int i=0;i<n;i++)
cout<<dpo[i]<<" ";
cout<<"\n";
for(int i=0;i<n;i++)
cout<<dpi[i]<<" ";
cout<<"\n";*/
int res=1e9;
for(int i=0;i<n;i++){
int r=dpi[dpo[dpj[i]]];
//cout<<i<<" "<<r<<"\n";
if(r!=n){
res=min(res,r-i+1-3*k);
}
}
if(res==1e9)
res=-1;
cout<<res<<"\n";
}
int main(){
ios::sync_with_stdio(false);cout.tie(NULL);cin.tie(NULL);
//freopen("/Users/jakob/cp_testing/test.txt","r",stdin);freopen("output.out","w",stdout);
int t=1;//cin>>t;
while(t--)solve();
return 0;
}
/*
*/
