#include <bits/stdc++.h>
#define ll long long
#define fi first
#define se second
#define sz(x) (int)x.size()
#define ALL(v) v.begin(),v.end()
#define MASK(k) (1LL << (k))
#define BIT(x, i) (((x) >> (i)) & 1)
#define oo (ll)1e18 + 10
#define INF (ll)1e9
#define MOD (ll)(998244353)
using namespace std;
template<class T1, class T2>
bool maximize(T1 &a, T2 b){if(a < b){a = b; return true;} return false;}
template<class T1, class T2>
bool minimize(T1 &a, T2 b){if(a > b){a = b; return true;} return false;}
template<class T1, class T2>
void add(T1 &a, T2 b){a += b; if(a >= MOD) a -= MOD;}
template<class T1, class T2>
void sub(T1 &a, T2 b){a -= b; if(a < 0) a += MOD;}
template<class T>
void cps(T &v){sort(ALL(v)); v.resize(unique(ALL(v)) - v.begin());}
const int MAX = 2e5 + 10;
int N, K;
string s;
int prefix[MAX], suffix[MAX];
void solve(){
cin >> N >> K >> s;
s = "*" + s + "*";
int j = 0, cnt = 0;
for(int i = 1; i <= N; i++){
cnt += (s[i] == 'J');
while(cnt > K || (cnt == K && s[j] != 'J')){
cnt -= (s[j] == 'J');
j++;
}
if(cnt == K) prefix[i] = j;
}
j = N + 1;
cnt = 0;
for(int i = N; i >= 1; i--){
cnt += (s[i] == 'I');
while(cnt > K || (cnt == K && s[j] != 'I')){
cnt -= (s[j] == 'I');
j--;
}
if(cnt == K) suffix[i] = j;
}
j = cnt = 0;
int ans = INF;
for(int i = 1; i < N; i++){
cnt += (s[i] == 'O');
while(cnt > K || (cnt == K && s[j] != 'O')){
cnt -= (s[j] == 'O');
j++;
}
if(cnt == K && prefix[j - 1] != 0 && suffix[i + 1] != 0){
minimize(ans, suffix[i + 1] - prefix[j - 1] + 1 - 3 * K);
}
}
cout << (ans >= INF ? -1 : ans) << '\n';
}
int main(){
ios_base::sync_with_stdio(0); cin.tie(0);
// freopen("task.inp","r",stdin);
// freopen("task.out","w",stdout);
int T = 1;
// cin >> T;
for(; T; T--) solve();
return 0;
}
