#include <bits/stdc++.h>
using namespace std;
const int inf = 1e9;
vector<int> po, jp, ip;
vector<pair<int, int>> J, I;
int n, k;
string s;
int solve(){
int ans = inf;
for (auto [il, ir] : I){
while (!J.empty()){
if (po[il] - po[J.back().second] >= k){
ans = min(ans, il - 2*k - J.back().first);
break;
}
else{
J.pop_back();
}
}
if (J.empty()){
return ans;
}
}
return ans;
}
int main () {
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cin >> n >> k >> s;
po.resize(n+1, 0);
for (int i=0; i<n; i++){
if (s[i] == 'J'){
jp.push_back(i+1);
po[i+1] = po[i];
}
else if (s[i] == 'I'){
ip.push_back(i+1);
po[i+1] = po[i];
}
else{
po[i+1] = po[i]+1;
}
}
for (int l=0, r=l+k-1; r<jp.size(); l++, r++){
J.push_back({jp[l], jp[r]});
}
for (int l=0, r=l+k-1; r<ip.size(); l++, r++){
I.push_back({ip[l], ip[r]});
}
reverse(J.begin(), J.end());
reverse(I.begin(), I.end());
int ans = solve();
if (ans == inf) cout << "-1\n";
else cout << ans << '\n';
return 0;
}
