#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define pll pair<ll,ll>
#define fs first
#define sc second
const int inf = 1e9;
void solve(){
int n,k;
cin>>n>>k;
string s;
cin>>s;
deque<char> dq;
for(auto &i:s)dq.push_back(i);
while(!dq.empty()&&dq.front() != 'J')dq.pop_front();
while(!dq.empty()&&dq.back() != 'I')dq.pop_back();
n = dq.size();
int ans = inf;
int jp[n+1] = {},ip[n+1] = {},op[n+1] = {};
int pref = 0;
int p = 0;
//for(auto &i:dq)cout<<i;cout<<endl;
for(int i = 0;i<n;i++){
if(p<=i){
p = i+1;
if(dq[i] == 'J')pref = 1;
else pref = 0;
}
while(p<n&&pref<k){
if(dq[p] == 'J')pref++;
p++;
}
if(pref<k)jp[i] = -1;
else jp[i] = p;
if(dq[i] == 'J')pref--;
}
pref = 0;
p = 0;
for(int i = 0;i<n;i++){
if(p<=i){
p = i+1;
if(dq[i] == 'O')pref = 1;
else pref = 0;
}
while(p<n&&pref<k){
if(dq[p] == 'O')pref++;
p++;
}
if(pref<k)op[i] = -1;
else op[i] = p;
if(dq[i] == 'O')pref--;
}
pref = 0;
p = 0;
for(int i = 0;i<n;i++){
if(p<=i){
p = i+1;
if(dq[i] == 'I')pref = 1;
else pref = 0;
}
while(p<n&&pref<k){
if(dq[p] == 'I')pref++;
p++;
}
if(pref<k)ip[i] = -1;
else ip[i] = p;
if(dq[i] == 'I')pref--;
}
for(int i = 0;i<n;i++){
int r = i;
r = jp[r];
if(r == -1)continue;
r = op[r];
if(r == -1)continue;
r = ip[r];
if(r == -1)continue;
ans = min(ans,r-i-k*3);
}
cout<<(ans == inf?-1:ans)<<'\n';
}
int main(){
ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
int t= 1;
while(t--)solve();
}
