#include<bits/stdc++.h>
#define fi first
#define se second
#define int long long
using namespace std;
using ll = long long;
using ii = pair<int, int>;
using aa = array<int,3>;
const int N = 1e6+5;
int n, k, ans = 1e18;
int pre[3][N];
string s;
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);cout.tie(NULL);
cin >> n >> k >> s;
while (!s.empty() && s.back() != 'I') s.pop_back();
reverse(s.begin(),s.end());
while (!s.empty() && s.back() != 'J') s.pop_back();
reverse(s.begin(),s.end());
if ((int)s.size() == 0) return cout << -1,0;
n = (int)s.size();
s = ' ' + s;
for (int i = 1; i <= n; i++){
for (int j = 0; j <= 2; j++){
pre[j][i] = pre[j][i-1];
}
if (s[i] == 'J') pre[0][i]++;
if (s[i] == 'O') pre[1][i]++;
if (s[i] == 'I') pre[2][i]++;
}
for (int i = 1; i <= n; i++){
int J = lower_bound(pre[0] + 1,pre[0] + 1 + n,pre[0][i-1] + k) - pre[0];
int O = lower_bound(pre[1] + J,pre[1] + 1 + n,pre[1][J] + k) - pre[1];
int I = lower_bound(pre[2] + O,pre[2] + 1 + n,pre[2][O] + k) - pre[2];
if (max({J,O,I}) <= n){
ans = min(ans,I-i+1-k*3);
}
}
cout << (ans == (int)1e18 ? -1 : ans);
return 0;
}
