#include <bits/stdc++.h>
#define ll long long
#define ull unsigned ll
#define pb push_back
#define pa pair<int, int>
#define pall pair<ll, int>
#define fi first
#define se second
#define TASK "test"
#define Size(x) (int) x.size()
#define all(x) x.begin(), x.end()
using namespace std;
template<typename T1, typename T2> bool mini (T1 &a, T2 b) {if(a > b) a = b; else return 0; return 1;}
template<typename T1, typename T2> bool maxi (T1 &a, T2 b) {if(a < b) a = b; else return 0; return 1;}
const int MOD = 1e9 + 7;
const int LOG = 20;
const int maxn = 2e5 + 7;
const ll oo = 1e9 + 69;
int n, k, f[maxn][3], nxt[3][maxn];
char a[maxn];
int convert(char ch) {
if(ch == 'J') return 0;
if(ch == 'O') return 1;
return 2;
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
//freopen(TASK".inp", "r", stdin);
//freopen(TASK".out", "w", stdout);
cin>>n>>k;
for(int i = 1; i <= n; i++) {
cin>>a[i];
for(int j = 0; j < 3; j++) f[i][j] = f[i - 1][j];
f[i][convert(a[i])]++;
}
for(int t = 0; t < 3; t++) {
int j = n;
nxt[t][n + 1] = n + 1;
for(int i = n; i >= 1; i--) {
nxt[t][i] = n + 1;
while(f[j - 1][t] - f[i - 1][t] >= k) j--;
if(f[j][t] - f[i - 1][t] == k) nxt[t][i] = j;
}
}
int ans = oo;
for(int i = 1; i <= n; i++) {
int p = i;
for(int j = 0; j < 3; j++) p = nxt[j][p];
if(p == n + 1) break;
mini(ans, p - i + 1 - 3 * k);
}
cout<<(ans == oo ? -1:ans);
return 0;
}
