#include <iostream>
#include <algorithm>
using namespace std;
const int N = 2e5 + 10;
int J[N], I[N], a[N], inf = 1e9;
int main(){
int n, k, Ans = N;
cin>>n>>k;
string s;
cin>>s;
s = '0' + s;
J[0] = inf;
for (int i=1, cnt = 0, l = 0;i<=n;i++){
cnt += (s[i] == 'J');
while (cnt - (s[l + 1] == 'J') >= k)
l++, cnt -= (s[l] == 'J');
J[i] = (cnt == k ? (i - l - k) : inf);
J[i] = min(J[i], J[i-1] + 1);
}
I[n+1] = inf;
for (int i=n, cnt = 0, r = n + 1;i>=1;i--){
cnt += (s[i] == 'I');
while (cnt - (s[r - 1] == 'I') >= k)
r--, cnt -= (s[r] == 'I');
I[i] = (cnt == k ? (r - i - k) : inf);
I[i] = min(I[i], I[i+1] + 1);
}
for (int i=n, cnt = 0, r = n + 1;i>=1;i--){
cnt += (s[i] == 'O');
while (cnt - (s[r - 1] == 'O') >= k)
r--, cnt -= (s[r] == 'O');
if (cnt == k)
Ans = min(Ans, r - i + J[i-1] + I[r] - k);
}
if (Ans == N)
Ans = -1;
cout<<Ans<<'\n';
}
