#include <bits/stdc++.h>
using namespace std;
#define all(x) x.begin(), x.end()
#define srtl(x)sort(all(x))
#define srtg(x)sort((x).begin(),(x).end(),greater<>())
#define rev(x) reverse(all(x))
#define lb(x,y) lower_bound(x.begin(),x.end(),y)-x.begin()
#define ub(x,y) upper_bound(x.begin(),x.end(),y)-x.begin()
#define ios ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define f first
#define s second
#define pb push_back
#define ins insert
#define next next_permutation
#define _b __builtin_popcount
#define ve vector
#define pii pair<int,int>
#define piii pair<int,pii>
#define vi vector<int>
#define vii vector<pii>
#define viii vector<piii>
#define vvi vector<vi>
#define vs vector<string>
#define vb vector<bool>
#define pV(x)for(auto i:x)cout<<i<<" ";
#define geta(a){for(auto &i:a)cin>>i;}
#define fr(n)for(int i=0;i<n;i++)
#define Fr(n)for(int i=n-1;i>=0;i--)
#define suma(a)accumulate(a.begin(),a.end(),0LL)
#define br(x){cout<<x<<"\n";return;}
#define yesn cout<<"YES\n";
#define yes()br("YES");
#define no(){br("NO")}
#define alice() br("Alice");
#define bob() br("Bob");
#define cn continue;
#define cint const int
#define int long long
int mod=1000000007;
cint mod1=100000007;
cint mod2=998244353;
int ax[8]={0,1,0,-1,-1,1,1,-1};
int ay[8]={1,0,-1,0,1,-1,1,-1};
//bool on(int x,int y){return (x>=0&&x<n&&y>=0&&y<m);}
//int euc(int a,int b,int c,int d){return abs(a-c)+abs(b-d);}
int bp(int x,int y){
if(y==0)return 1;
int r=bp(x,y/2);
return (y&1)?r*r%mod*x%mod:r*r%mod;
}
int bpm(int x,int y){
if(y==0)return 1;
int r=bp(x,y/2);
return (y&1)?r*r*x:r*r;
}
cint N=200006;
int F[N+1],I[N+1];
int cmb(int n,int k){
return (((F[n]*I[k])%mod)*I[n-k])%mod;
}
int cinn(){
int n;
cin>>n;
return n;
}
#define w cinn()
void solve(){
int n,k;
cin>>n>>k;
string x;
cin>>x;
vi pj(n,0),po(n,0),pi(n,0);
for(int i=0;i<n;i++){
if(x[i]=='J')pj[i]++;
if(x[i]=='O')po[i]++;
if(x[i]=='I')pi[i]++;
if(i){
pj[i]+=pj[i-1];
po[i]+=po[i-1];
pi[i]+=pi[i-1];
}
}
vi prj(n+1,1e18);
for(int i=n-1;i>=0;i--){
prj[i]=prj[i+1];
if(x[i]=='J')prj[i]=i;
}
int res=1e18;
for(int i=0;i<n;i++){
int mi=i-1,ma=n,me;
int p1=1e18,p2=1e18,p3=1e18;
while(ma-mi!=1){
me=(ma+mi)/2;
if(pj[me]-(i?(pj[i-1]):0)>=k){
p1=min(p1,me);
ma=me;
}
else mi=me;
}
if(p1==1e18)continue;
mi=p1,ma=n;
while(ma-mi!=1){
me=(ma+mi)/2;
if(po[me]-po[p1]>=k){
p2=min(p2,me);
ma=me;
}
else mi=me;
}
mi=p2,ma=n;
if(p2==1e18)continue;
while(ma-mi!=1){
me=(ma+mi)/2;
if(pi[me]-pi[p2]>=k){
p3=min(p3,me);
ma=me;
}
else mi=me;
}
if(p3==1e18)continue;
res=min(res,(p3-(prj[i])+1 - (3*k) ));
}
if(res==1e18)res=-1;
cout<<res;
}
int32_t main(){
ios;
int t=1;
//cin>>t;
/*
F[0]=1;
for(int i=1;i<=N;i++){
F[i]=(i*F[i-1])%mod;
}
I[N]=(bp(F[N],mod-2))%mod;
for(int i=N-1;i>=0;i--){
I[i]=(I[i+1]*(i+1))%mod;
}
//*/
while(t--){
solve();
}
return 0;
}
