#include <bits/stdc++.h>
using namespace std;
const int maxn = 2e5 + 5;
int n, k;
int a[maxn];
int pfs[maxn][3];
int bs(int l, int idx)
{
int lo = l, hi = n+1;
while (lo < hi)
{
int mid = lo + (hi - lo) / 2;
if (pfs[mid][idx] - pfs[l-1][idx] >= k) hi = mid;
else lo = mid + 1;
}
return (lo >= n+1 ? -1 : lo);
}
int main()
{
ios::sync_with_stdio(false); cin.tie(0);
cin >> n >> k;
for (int i = 1; i <= n; ++i) {
char c; cin >> c;
for (int j = 0; j < 3; ++j) pfs[i][j] = pfs[i-1][j];
if (c == 'J') a[i] = 0;
else if (c == 'O') a[i] = 1;
else a[i] = 2;
pfs[i][a[i]]++;
}
int ops = INT_MAX;
for (int i = 1; i <= n; ++i)
{
int pos1 = bs(i, 0);
int pos2 = bs(pos1+1, 1);
int pos3 = bs(pos2+1, 2);
// cout << pos1 << ' ' << pos2 << ' ' << pos3 << '\n';
if (pos1 != -1 && pos2 != -1 && pos3 != -1) ops = min(ops, pos3-i+1-3*k);
}
cout << (ops == INT_MAX ? -1 : ops);
return 0;
}
