#include <bits/stdc++.h>
using namespace std;
int N, K, ans=1e9;
string S, T="JOI";
int psum[3][200100];
int binser(int start, int id) {
int L=start, R=N;
int res = 0;
while(L<=R) {
int mid = (L+R)/2;
if (psum[id][mid]-psum[id][start-1] >= K) {
res = mid;
R = mid-1;
} else {
L = mid+1;
}
}
return res;
}
int main () {
cin >> N >> K;
cin >> S;
S = "."+S;
for(int i=1; i<=N; i++) {
for(int j=0; j<=2; j++) {
psum[j][i] = psum[j][i-1] + (S[i]==T[j]);
}
}
for(int i=1; i<=N; i++) {
int x = i-1;
bool valid = true;
for(int j=0; j<=2; j++) {
x = binser(x+1,j);
if (x==0) {
valid = false;
break;
}
}
if (!valid) continue;
ans = min(ans, x-i+1-3*K);
}
if (ans==1e9) ans = -1;
cout << ans << endl;
}
