#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int maxn = 2e5+5, maxk = 18;
int n, K;
string s;
int jJ[maxk][maxn], jO[maxk][maxn], jI[maxk][maxn];
int walk(int i, int ar[][maxn]) {
for (int k = maxk-1; k >= 0; k--) {
if (K & (1<<k)) {
i = ar[k][i];
}
if (i == -1) return -1;
}
return i;
}
int main() {
ios_base::sync_with_stdio(false); cin.tie(NULL);
cin >> n >> K >> s;
memset(jJ,-1,sizeof jJ);
memset(jO,-1,sizeof jO);
memset(jI,-1,sizeof jI);
for (int i = n-1; i >= 0; i--) {
jJ[0][i] = s[i] == 'J' ? i+1 : jJ[0][i+1];
for (int k = 1; k < maxk; k++) {
jJ[k][i] = jJ[k-1][i] == -1 ? -1 : jJ[k-1][jJ[k-1][i]];
}
}
for (int i = n-1; i >= 0; i--) {
jO[0][i] = s[i] == 'O' ? i+1 : jO[0][i+1];
for (int k = 1; k < maxk; k++) {
jO[k][i] = jO[k-1][i] == -1 ? -1 : jO[k-1][jO[k-1][i]];
}
}
for (int i = n-1; i >= 0; i--) {
jI[0][i] = s[i] == 'I' ? i+1 : jI[0][i+1];
for (int k = 1; k < maxk; k++) {
jI[k][i] = jI[k-1][i] == -1 ? -1 : jI[k-1][jI[k-1][i]];
}
}
int ans = 2e9;
for (int i = 0; i < n; i++) {
int r = i;
r = walk(r,jJ);
if (r == -1) continue;
r = walk(r,jO);
if (r == -1) continue;
r = walk(r,jI);
if (r == -1) continue;
int tr = (r-i)-3*K;
ans = min(ans,tr);
}
cout << (ans < 2e9 ? ans : -1) << '\n';
}
