#include<bits/stdc++.h>
using namespace std;
const int MX = 2e5 + 5;
int prej[MX], preo[MX], prei[MX];
int get(int l, int r, int *a) {
if(l>r) return 0;
return a[r] - a[l-1];
}
string s;
void PlayGround() {
int n, k;
cin>>n>>k;
cin>>s;
s = "$" + s;
for(int i=1; i<=n; ++i) {
if(s[i]=='J') prej[i] = 1;
else if(s[i]=='O') preo[i] = 1;
else prei[i] = 1;
prej[i] += prej[i-1];
preo[i] += preo[i-1];
prei[i] += prei[i-1];
}
int ans = INT_MAX;
for(int i=1; i<=n; ++i) {
int j = i;
if(get(j, n, prej)<k) break;
int l = i, r = n;
while(l<r) {
int mid = (l+r)/2;
if(get(j, mid, prej)>=k) r = mid;
else l = mid+1;
}
++l, r = n, j = l;
if(get(j, n, preo)<k) break;
while(l<r) {
int mid = (l+r)/2;
if(get(j, mid, preo)>=k) r = mid;
else l = mid+1;
}
++l, r = n, j = l;
if(get(j, n, prei)<k) break;
while(l<r) {
int mid = (l+r)/2;
if(get(j, mid, prei)>=k) r = mid;
else l = mid+1;
}
ans = min(ans, l-i+1 - k*3);
}
if(ans==INT_MAX) ans = -1;
cout<<ans<<'\n';
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
PlayGround();
return 0;
}
