#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define MAXN (1000005)
int main(){
ios_base::sync_with_stdio(false);cin.tie(0);
ll N,K;
cin>>N>>K;
string s;
cin>>s;
ll Opsum[N];
ll cur = 0;
for(ll i = 0;i < N;i++){
if(s[i] == 'O'){
cur++;
}
Opsum[i] = cur;
}
ll Jpre[N];
cur = 0;
ll ptr = 0;
for(ll r = 0;r < N;r++){
if(s[r] == 'J'){
cur++;
}
while(ptr < r && (cur > K || s[ptr] != 'J')){
if(s[ptr] == 'J') cur--;
ptr++;
}
if(cur >= K) Jpre[r] = ptr;
else Jpre[r] = -1;
}
ll Isuf[N];
cur = 0;
ptr = N - 1;
for(ll l = N - 1;l >= 0;l--){
if(s[l] == 'I'){
cur++;
}
while(ptr > l && (cur > K || s[ptr] != 'I')){
if(s[ptr] == 'I') cur--;
ptr--;
}
if(cur >= K) Isuf[l] = ptr;
else Isuf[l] = -1;
}
ll L = 1;
ll R = 1;
bool ok = 0;
ll ans = 1e18;
while(R < N - 1){
while(L < R && Opsum[R] - Opsum[L] >= K){
L++;
}
if(Jpre[L - 1] != -1 && Isuf[R + 1] != -1 && Opsum[R] - Opsum[L - 1] >= K){
ok = 1;
ans = min(ans,(Isuf[R + 1] - Jpre[L - 1] + 1) - 3 * K);
}
R++;
}
if(!ok){
cout<<-1<<'\n';
}else{
cout<<ans<<'\n';
}
}
