#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
using namespace std;
int main()
{
int n, k;
cin >> n >> k;
string s;
cin >> s;
vector<int> preJ(n, 0), preO(n, 0), preI(n, 0);
preJ[0] = (s[0] == 'J');
preO[0] = (s[0] == 'O');
preI[0] = (s[0] == 'I');
for (int i = 1; i < n; i++)
{
preJ[i] = preJ[i - 1] + (s[i] == 'J');
preO[i] = preO[i - 1] + (s[i] == 'O');
preI[i] = preI[i - 1] + (s[i] == 'I');
}
int ans = n + 1;
for (int i = 0; i < n; i++)
{
int x = lower_bound(preJ.begin(), preJ.end(), i + 1) - preJ.begin();
int J = lower_bound(preJ.begin(), preJ.end(), i + k) - preJ.begin();
if (J >= n)
continue;
int O = lower_bound(preO.begin(), preO.end(), preO[J] + k) - preO.begin();
if (O >= n)
continue;
int I = lower_bound(preI.begin(), preI.end(), preI[O] + k) - preI.begin();
if (I >= n)
continue;
ans = min(ans, I - x + 1 - 3 * k);
}
cout << (ans == n + 1 ? -1 : ans) << endl;
return 0;
}
