#include <bits/stdc++.h>
using namespace std;
constexpr int MAX = 2e5 + 69;
constexpr int inf = 1e9 + 69;
int n, k;
char s[MAX];
int bit[3][MAX];
inline void add(int y, int x, int val)
{
for (int i = x; i < MAX; i += (i & -i))
bit[y][i] += val;
}
inline int sum(int y, int x)
{
int res = 0;
for (int i = x; i > 0; i -= (i & -i))
res += bit[y][i];
return res;
}
int main()
{
ios :: sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cin >> n >> k >> (s+1);
for (int i = 1; i <= n; ++i)
{
if (s[i] == 'J') add(0, i, 1);
if (s[i] == 'O') add(1, i, 1);
if (s[i] == 'I') add(2, i, 1);
}
int res = inf;
for (int i = 0; i <= n; ++i)
{
int x = i+1;
for (int y = 0; y < 3; ++y)
{
int l = x, r = n+1;
while (l < r)
{
int mid = (l + r) >> 1;
if (sum(y, mid) - sum(y, x-1) < k) l = mid + 1;
else r = mid;
}
x = l;
}
if (x == n+1) break;
res = min(res, x-i - 3*k);
}
cout << (res == inf ? -1 : res) << '\n';
return 0;
}
