// #pragma optimize ("g",on)
// #pragma GCC optimize ("inline")
// #pragma GCC optimize ("Ofast")
// #pragma GCC optimize ("unroll-loops")
// #pragma GCC optimize ("03")
#include <bits/stdc++.h>
#define pb push_back
#define SS ios_base::sync_with_stdio(0);cin.tie(nullptr);cout.tie(nullptr);
#define int long long
#define all(v) v.begin(),v.end()
using namespace std;
mt19937 rnd(chrono::steady_clock::now().time_since_epoch().count());
const int N = 5e5 + 1, inf = 1e9, mod = 998244353;
void solve(){
int n, k;
cin >> n >> k;
string s;
cin >> s;
s = '!' + s;
int ans = 0, cnt = 0, l = 1, r = n;
for(int i = 1; i <= n; i++){
if(cnt == k) break;
l = i;
if(s[i] == 'J'){
cnt++;
}
else{
ans += (cnt > 0);
}
}
if(cnt != k){
cout << -1 << '\n';
return;
}
cnt = 0;
for(int i = n; i >= 1; i--){
if(cnt == k) break;
r = i;
if(s[i] == 'I'){
cnt++;
}
else{
ans += (cnt > 0);
}
}
if(cnt != k){
cout << -1 << '\n';
return;
}
cnt = 0;
for(int i = l + 1; i < r; i++){
if(s[i] != 'O') ans++;
else{
if(cnt == k) ans++;
else cnt++;
}
}
if(cnt != k){
cout << -1 << '\n';
return;
}
cout << ans;
}
signed main(){
SS
// freopen("trains.in", "r", stdin);
// freopen("trains.out", "w", stdout);
int t = 1;
// cin >> t;
while(t--){
solve();
}
}
