#include<bits/stdc++.h>
using namespace std;
#define task "a"
#define se second
#define fi first
#define ll long long
#define ii pair<ll, ll>
const long mxN = 2e5 + 7, inf = 1e9 + 7;
int lst[3][mxN], n, k;
queue<int> q[3];
string s;
char chr[3] = {'J', 'O', 'I'};
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
//freopen(task".INP", "r", stdin);
//freopen(task".OUT", "w", stdout);
cin >> n >> k;
cin >> s;
s = ' ' + s;
int ans = inf;
for (int i = 1; i <= n; i++)
{
for (int j = 0; j < 3; j++)
{
lst[j][i] = lst[j][i - 1];
if (s[i] != chr[j])
continue;
q[j].push(i);
if (q[j].size() > k)
q[j].pop();
if (q[j].size() == k)
lst[j][i] = q[j].front();
}
int cur = i;
for (int j = 2; j >= 0; j--)
cur = lst[j][cur];
if (cur)
ans = min(ans, i - cur + 1);
}
if (ans == inf)
cout << -1;
else
cout << ans - 3 * k;
}
