#include <bits/stdc++.h>
using namespace std;
#define int long long
#define ll long long
#define ld long double
#define all(v) v.begin(),v.end()
#define ar array
const int M = 1e6;
const int N = 4e5 + 20;
const int LOG = 61;
const int INF = 1e18;
const int MOD = 1e9 + 7;
const ld EPS = 1e-9;
inline void mm(int &x){x = (x % MOD + MOD) % MOD;}
inline void chmin(int &x, int y){x = min(x, y);}
inline void chmax(int &x, int y){x = max(x, y);}
#pragma GCC optimize("unroll-loops,O3")
void orz(){
int n, k;
cin>>n>>k;
string s;
cin>>s;
vector<int> v[3];
for(int i = 0;i < n;i++){
if(s[i] == 'J')v[0].push_back(i);
if(s[i] == 'O')v[1].push_back(i);
if(s[i] == 'I')v[2].push_back(i);
}
for(int i = 0;i < 3;i++)v[i].push_back(n);
int ans = INF;
for(int i = 0;i < n;i++){
if(s[i] != 'J')continue;
int j = i;
for(int x = 0;x < 3;x++){
j = lower_bound(all(v[x]), j) - v[x].begin() + k - 1;
j = min(j, (int)v[x].size() - 1);
j = v[x][j];
}
if(j < n)ans = min(ans, j - i + 1);
}
if(ans >= INF)cout<<-1;
else cout<<ans - 3 * k;
}
signed main(){ios_base::sync_with_stdio(false);cin.tie(0);
int t = 1;
//cin>>t;
while(t--)orz();
}
