#include <bits/stdc++.h>
using namespace std;
int vj[200001], vo[200001], vi[200001];
int lj, lo, li;
int main()
{
int n, k;
cin >> n >> k;
string s;
cin >> s;
for(int i = 0; i < n; i++)
{
if(s[i] == 'J')
vj[++lj] = i;
if(s[i] == 'O')
vo[++lo] = i;
if(s[i] == 'I')
vi[++li] = i;
}
int pj = 1;
int po = 1;
int pi = 1;
int ans = n+1;
while(pj + k - 1 <= lj && po + k - 1 <= lo && pi + k - 1 <= li)
{
while(po + k - 1 <= lo && vo[po] <= vj[pj + k - 1])
po++;
if(po + k - 1 <= lo)
while(pi + k - 1 <= li && vi[pi] <= vo[po + k - 1])
pi++;
if(pj + k - 1 <= lj && po + k - 1 <= lo && pi + k - 1 <= li)
{
int fi = vj[pj];
int lst = vi[pi + k - 1];
ans = min(ans, (lst - fi + 1) - 3 * k);
}
pj++;
}
if(ans == n+1)
cout << -1;
else
cout << ans;
return 0;
}
