#include <bits/stdc++.h>
using namespace std;
int main() {
int n, k;
cin >> n >> k;
string s;
cin >> s;
vector<int> jpos(n+1), opos(n+1), ipos(n+1);
for (int l = 1; l <= n; l++) {
jpos[l] = jpos[l-1], ipos[l] = ipos[l-1], opos[l] = opos[l-1];
if (s[l-1] == 'J') jpos[l]++;
if (s[l-1] == 'O') opos[l]++;
if (s[l-1] == 'I') ipos[l]++;
}
int ans = INT_MAX;
for (int j = 0; j < n; j++) {
int x = j;
int a = lower_bound(jpos.begin()+x, jpos.end(), jpos[x]+k) - jpos.begin();
if (a > n) break;
int b = lower_bound(opos.begin()+a, opos.end(), opos[a]+k) - opos.begin();
if (b > n) break;
int c = lower_bound(ipos.begin()+b, ipos.end(), ipos[b]+k) - ipos.begin();
if (c > n) break;
ans = min(ans, (c-x)-(3*k));
}
if (ans == INT_MAX) ans = -1;
cout << ans << '\n';
return 0;
}
