#include <bits/stdc++.h>
#define F first
#define S second
using namespace std;
typedef pair<long long,long long> ii;
const int N = 6e5+5;
int n,k;
string a;
int J[N],O[N],I[N];
int q;
int howO[N],howI[N],howJ[N];
int main()
{
cin.tie(0); ios_base::sync_with_stdio(0);
cin >> n >> k;
cin >> a;
q=0;
for (int i=1;i<=n+n+n;i++) J[i] = O[i] = I[i] = -1;
for (int i=0;i<n;i++) if (a[i]=='J') {
++q;
J[q] = i;
}
q=0;
for (int i=0;i<n;i++) if (a[i]=='O') {
++q;
O[q] = i;
}
for (int i=0;i<n;i++) {
if (a[i]=='O') howO[i]++;
if (i>0) howO[i] +=howO[i-1];
}
q=0;
for (int i=0;i<n;i++) if (a[i]=='I') {
++q;
I[q] = i;
}
for (int i=0;i<n;i++) {
if (a[i]=='I') howI[i]++;
if (i>0) howI[i] +=howI[i-1];
}
for (int i=0;i<n;i++) {
if (a[i]=='J') howJ[i]++;
if (i>0) howJ[i] +=howJ[i-1];
}
bool y = false;
int res = n;
for (int i=k;i<=n;i++) {
if (J[i]==-1) i = n+1;
else {
int p1 = i-(k-1);
p1 = J[p1];
int p2 = J[i];
int os = howO[J[i]];
int p3 = p2+1;
int p4 = O[os+k];
if (p4 == -1) {
} else {
int is = howI[O[os+k]];
int p5 = p4+1;
int p6 = I[is+k];
if (p6 == -1) {
} else {
y = true;
int x = howI[p2]-howI[p1] + howO[p2]-howO[p1];
int xx = howJ[p4]-howJ[p3] + howI[p4]-howI[p3];
int xxx = howJ[p6]-howJ[p5] + howO[p6]-howO[p5];
res = min(res,x+xx+xxx);
}
}
}
}
if (y) cout << res;
else cout << -1;
return 0;
}
