#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define ARRS ((ll)(4e5+100))
#define fr first
#define sc second
#define pb push_back
#define MX ((ll)(1e9+100))
#define MAX ((ll)(1e18+100))
ll n;
pair<ll,ll> a[ARRS];
ll b[ARRS];
ll ans[ARRS];
ll pl[ARRS];
ll pr[ARRS];
int main(){
ll n,m;
string s;
cin>>n>>m;
cin>>s;
ll l=0,r=0,c=0;
for(r=0; r<n&&c!=m; r++){
c+=s[r]=='J';
pl[r]=MAX;
}
if(c<m){
cout<<-1;
return 0;
}
while(s[l]!='J')l++;
for(;r<n;r++){
pl[r]=r-l-m;
if(s[r]=='J'){
l++;
while(s[l]!='J')l++;
}
}
reverse(s.begin(),s.end());
l=0,r=0,c=0;
for(r=0; r<n&&c!=m; r++){
c+=s[r]=='I';
pr[n-r-1]=MAX;
}
if(c<m){
cout<<-1;
return 0;
}
while(s[l]!='I')l++;
for(;r<n;r++){
pr[n-r-1]=r-l-m;
if(s[r]=='I'){
l++;
while(s[l]!='I')l++;
}
}
reverse(s.begin(),s.end());
l=0,r=0,c=0;
for(r=0; r<n&&c!=m; r++){
c+=s[r]=='O';
}
if(c<m){
cout<<-1;
return 0;
}
while(s[l]!='O')l++;
ll mn=MAX;
for(;r<n;r++){
mn=min(mn, pl[l]+pr[r-1]+r-l-m);
if(s[r]=='O'){
l++;
while(s[l]!='O')l++;
}
}
if(mn==MAX)
cout<<-1<<endl;
else
cout<<mn<<endl;
}
