#include <bits/stdc++.h>
using namespace std;
int jp[200007];
int op[200007];
int ip[200007];
int main()
{
int n, k;
string s;
cin >> n >> k >> s;
int jc = 0, oc = 0, ic = 0;
for(int i = 1; i <= n; i++)
{
if(s[i - 1] == 'J')
{
jc++;
jp[jc] = i;
}
else if(s[i - 1] == 'O')
{
oc++;
op[oc] = i;
}
else if(s[i - 1] == 'I')
{
ic++;
ip[ic] = i;
}
}
if(jc < k || oc < k || ic < k)
{
cout << -1 << endl;
return 0;
}
int ans = INT_MAX;
for(int i = 1; i + k - 1 <= jc; i++)
{
if(int(lower_bound(op + 1, op + oc + 1, jp[i + k - 1] + 1) - op) > oc - k + 1)
{
break;
}
if(lower_bound(ip + 1, ip + ic + 1, op[lower_bound(op + 1, op + oc + 1, jp[i + k - 1] + 1) - op + k - 1] + 1) - ip > ic - k + 1)
{
break;
}
ans = min(ans, ip[int(lower_bound(ip + 1, ip + ic + 1, op[lower_bound(op + 1, op + oc + 1, jp[i + k - 1] + 1) - op + k - 1] + 1) - ip) + k - 1] - jp[i] + 1 - 3 * k);
}
if(ans == INT_MAX)
{
cout << -1 << endl;
}
else
{
cout << ans << endl;
}
return 0;
}
