#include <iostream>
#include <vector>
#include <limits.h>
using namespace std;
const char el = '\n';
int main() {
ios_base::sync_with_stdio(false), cin.tie(nullptr);
int n, k; cin >> n >> k;
string s; cin >> s;
vector<int> J, O, I;
for(int i=0; i<n; i++)
switch(s[i]) {
case 'J': J.push_back(i); break;
case 'O': O.push_back(i); break;
case 'I': I.push_back(i); break;
}
int ij = 0, io = 0, ii = 0, jsz = J.size(), osz = O.size(), isz = I.size();
int endj, endo, sol = INT_MAX;
auto print_sol = [&] () {
if(sol==INT_MAX) cout << -1 << el;
else cout << sol << el;
return 0;
};
while(true) {
if(ij+k-1>=jsz) return print_sol();
endj = J[ij+k-1];
while(io<osz && O[io]<=endj) io++;
if(io+k-1>=osz) return print_sol();
endo = O[io+k-1];
while(ii<isz && I[ii]<=endo) ii++;
if(ii+k-1>=isz) return print_sol();
sol = min(sol, I[ii+k-1]-J[ij]+1-k-k-k);
ij++;
}
return print_sol();
}
