#include <bits/stdc++.h>
using namespace std;
const int maxn = 3e3+10;
const int inf = 1e9+10;
int n, k;
string a;
int dp[maxn][maxn][3];
int solve(int i, int j, int q)
{
if (q == 2 && j == k) return 0;
if (i == n) return inf;
if (dp[i][j][q] != -1) return dp[i][j][q];
int ans = inf;
if (q == 0 && j == 0) ans = min(ans, solve(i+1, 0, 0));
if (q == 0)
{
ans = min(ans, 1+solve(i+1, j, q));
if (a[i] == 'J')
{
if (j+1 < k) ans = min(ans, solve(i+1, j+1, q));
else ans = min(ans, solve(i+1, 0, 1));
}
}
else if (q == 1)
{
ans = min(ans, 1+solve(i+1, j, q));
if (a[i] == 'O')
{
if (j+1 < k) ans = min(ans, solve(i+1, j+1, q));
else ans = min(ans, solve(i+1, 0, 2));
}
}
else
{
ans = min(ans, 1+solve(i+1, j, q));
if (a[i] == 'I')
ans = min(ans, solve(i+1, j+1, q));
}
return dp[i][j][q] = ans;
}
int main(void)
{
cin >> n >> k >> a;
memset(dp, -1, sizeof dp);
int ans = solve(0, 0, 0);
if (ans == inf) cout << "-1\n";
else cout << ans << "\n";
}
