#include <bits/stdc++.h>
using namespace std;
#define __Master_Moon__ int main()
#define ll long long
#define el "\n"
#define base 300
#define fi first
#define sq(x) (x)*(x)
#define se second
#define pub push_back
#define puf push_front
#define pii pair <int, int>
#define pll pair <long long, long long>
#define piii pair <long long, pair <int, int>>
#define iiii pair <int, pair <int, pair <int, int>>>
#define plll pair <long long, pair <long long, long long>>
#define FOR(i, a, b) for(int i = (a);i <=(b);i++)
#define FO(i, a, b) for(int i = (a);i >= (b);i--)
#define REP(i, n) for(int i = 0;i < (n);i++)
long const maxn = 2e5 + 5;
int p[maxn][2],n,k,ans = INT_MAX;
string s;
void solve()
{
cin >> n >> k >> s;
s = ' ' + s;
int j = 1,cnt = 0;
memset(p,-1,sizeof p);
FOR(i,1,n)
{
if(s[i] == 'J') cnt++;
while(cnt >= k)
{
if(s[j] == 'J')
{
cnt--;
if(cnt < k)
{
cnt++;
break;
}
}
j++;
}
if(cnt >= k) p[i][0] = j;
}
j = 1;
cnt = 0;
FOR(i,1,n)
{
if(s[i] == 'O') cnt++;
while(cnt >= k)
{
if(s[j] == 'O')
{
cnt--;
if(cnt < k)
{
cnt++;
break;
}
}
j++;
}
if(cnt >= k && p[j-1][0] != -1) p[i][1] = p[j-1][0];
}
j = 1;
cnt = 0;
FOR(i,1,n)
{
if(s[i] == 'I') cnt++;
while(cnt >= k)
{
if(s[j] == 'I')
{
cnt--;
if(cnt < k)
{
cnt++;
break;
}
}
j++;
}
if(cnt >= k && p[j-1][1] != -1) ans = min(ans,i-p[j-1][1]-3*k+1);
}
if(ans == INT_MAX) ans = -1;
cout << ans;
}
__Master_Moon__
{
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
solve();
return 0;
}
