#include <bits/stdc++.h>
using namespace std;
const int Nmax = 2e5 + 5;
int sI[Nmax], sJ[Nmax], sO[Nmax], N, K;
char a[Nmax];
int main()
{
// freopen("input", "r", stdin);
cin.sync_with_stdio(false); cin.tie(0);
cin >> N >> K;
cin >> (a+1);
int s, i, j, k, ans = 1e9;
for(i=1; i<=N; ++i) sJ[i] = sJ[i-1] + (a[i] == 'J');
for(i=1; i<=N; ++i) sO[i] = sO[i-1] + (a[i] == 'O');
for(i=1; i<=N; ++i) sI[i] = sI[i-1] + (a[i] == 'I');
i = j = k = 0;
for(s=0; s<N; ++s)
{
while(i <= N && sJ[i] - sJ[s] < K) ++i;
while(j <= N && sO[j] - sO[i] < K) ++j;
while(k <= N && sI[k] - sI[j] < K) ++k;
if(i <= N && j <= N && k <= N && k - s < ans)
ans = k - s;
}
ans -= 3 * K;
if(ans > N) ans = -1;
cout << ans << '\n';
return 0;
}
