#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 2e5+5;
int n, k;
int prefJ[N], prefO[N], prefI[N];
void read() {
cin >> n >> k;
prefJ[0] = prefO[0] = prefI[0] = 0;
for(int i = 1; i <= n; i++) {
char c; cin >> c;
prefJ[i] = prefJ[i-1] + (c == 'J');
prefO[i] = prefO[i-1] + (c == 'O');
prefI[i] = prefI[i-1] + (c == 'I');
}
}
int binserO(int l, int r) {
int d = l-1;
while(l < r) {
int mid = (l+r)/2;
if(prefO[mid] - prefO[d] < k) l = mid+1;
else r = mid;
}
return l;
}
int binserI(int l, int r) {
int d = l-1;
while(l < r) {
int mid = (l+r)/2;
if(prefI[mid] - prefI[d] < k) l = mid+1;
else r = mid;
}
return l;
}
void solve() {
int ans = 1e9;
int fp = 0, prv = 0;
for(int i = 1; i <= n; i++) {
if(prv < prefJ[i] && prefJ[i] >= k) {
while(prefJ[fp] <= prefJ[i] - k) fp++;
int mp = binserO(i+1, n+1);
if(mp == n+1) break;
int lp = binserI(mp+1, n+1);
if(lp == n+1) break;
ans = min(ans, lp - fp + 1 - 3*k);
}
prv = prefJ[i];
}
cout << ((ans==1e9)?(-1):ans) << '\n';
}
int main() {
ios::sync_with_stdio(0); cin.tie(0);
read(); solve();
return 0;
}
