#include<bits/stdc++.h>
#define fi first
#define se second
#define eb emplace_back
#define mp make_pair
using namespace std;
typedef long double ld;
typedef long long ll;
typedef unsigned long long ull;
typedef __int128 i128;
template<typename T,typename U>
T ceil(T x, U y) {return (x>0?(x+y-1)/y:x/y);}
template<typename T,typename U>
T floor(T x, U y) {return (x>0?x/y:(x-y+1)/y);}
template<class T,class S>
bool chmax(T &a,const S b) {return (a<b?a=b,1:0);}
template<class T,class S>
bool chmin(T &a,const S b) {return (a>b?a=b,1:0);}
int popcnt(int x) {return __builtin_popcount(x);}
int popcnt(ll x) {return __builtin_popcountll(x);}
int topbit(int x) {return (x==0?-1:31-__builtin_clz(x));}
int topbit(ll x) {return (x==0?-1:63-__builtin_clzll(x));}
int lowbit(int x) {return (x==0?-1:__builtin_ctz(x));}
int lowbit(ll x) {return (x==0?-1:__builtin_ctzll(x));}

#define rep(i,a,b) for(int i=(a);i<=(b);i++)
#define per(i,a,b) for(int i=(a);i>=(b);i--)
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<pii> vp;
int read() {
  int x=0,w=1; char c=getchar(); 
  while(!isdigit(c)) {if(c=='-') w=-1; c=getchar();}
  while(isdigit(c)) {x=x*10+(c-'0'); c=getchar();}
  return x*w;
}

const int N=2e5+9;
int lst[129],fj[N][25],fo[N][25],fi[N][25],n,k,ans;
char s[N];

void pre(int f[N][25],int x) {
  rep(h,1,20) f[x][h]=f[f[x][h-1]][h-1];
}
int jmp(int f[N][25],int x,int k) {
  rep(h,0,20) if(k&(1<<h)) x=f[x][h];
  return x;
}

signed main() {
  scanf("%d%d%s",&n,&k,s+1);
  ans=n+1;
  rep(i,1,n) {
    fj[i][0]=lst['J'];
    fo[i][0]=lst['O'];
    fi[i][0]=lst['I'];
    pre(fj,i), pre(fo,i), pre(fi,i);
    lst[s[i]]=i;
  }
  rep(i,1,n) {
    int pos=i;
    pos=jmp(fi,pos,k-(s[pos]=='I'));
    pos=jmp(fo,pos,k-(s[pos]=='O'));
    pos=jmp(fj,pos,k-(s[pos]=='J'));
    if(pos) ans=min(ans,i-pos+1-3*k);
  }
  if(ans==n+1) puts("-1");
  else printf("%d\n",ans);
  return 0;
}