#include <iostream>
#include <vector>
using namespace std;
int main()
{
int N, K;
cin >> N >> K;
vector<int> J(N+1, 0), O(N+1, 0), I(N+1, 0);
char c;
for(int i = 1; i <= N; i++)
{
cin >> c;
J[i] = J[i-1];
O[i] = O[i-1];
I[i] = I[i-1];
if(c == 'J') J[i]++;
else if(c == 'O') O[i]++;
else if(c == 'I') I[i]++;
}
vector<int> dpJ(N+1, 1e9), dpO(N+2, 1e9), dpI(N+1, 1e9);
int prev = 0;
for(int i = 1; i <= N; i++)
{
if(J[i] < K) continue;
while(J[i] - J[prev+1] >= K) prev++;
dpJ[i] = min(dpJ[i-1] + 1, i - prev - K);
}
// for(int i = 1; i <= N; i++) cout << dpJ[i] << ' ';
// cout << '\n';
prev = N;
for(int i = N; i >= 1; i--)
{
if(I[N] - I[i-1] < K) continue;
while(I[prev-1] - I[i-1] >= K) prev--;
dpI[i] = min(dpI[i+1] + 1, prev - (i-1) - K);
}
// for(int i = 1; i <= N; i++) cout << dpI[i] << ' ';
// cout << '\n';
int res = 1e9;
int j = 1;
for(int i = 1; i <= N; i++)
{
while(j <= N && O[j] - O[i-1] < K) j++;
if(j == N+1) break;
res = min(res, dpJ[i] + j-i-1 - K + dpI[j]);
}
if(res == 1e9) cout << "-1\n";
else cout << res << '\n';
}
