#include <bits/stdc++.h>
using namespace std;
#define ll long long
int cnt[3][200005], last[3][200005], n, k;
string s;
void proc(int id){
int lf = 0;
for(int rg = 1; rg <= n; rg ++){
while(cnt[id][rg] - cnt[id][lf] >= k) lf ++;
last[id][rg - 1] = lf - 1;
}
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
cin >> n >> k;
cin >> s;
for(int i = 0; i < n; i ++){
if(s[i] == 'J') cnt[0][i + 1]++;
if(s[i] == 'O') cnt[1][i + 1]++;
if(s[i] == 'I') cnt[2][i + 1]++;
}
for(int i = 1; i <= n; i ++) for(int j = 0; j < 3; j ++)
cnt[j][i] += cnt[j][i - 1];
for(int i = 0; i < 3; i ++) proc(i);
int ans = 2000000000;
for(int i = 0; i < n; i ++){
int nw = last[2][i];
nw --;
if(nw >= 0) nw = last[1][nw];
else nw = -1000000;
nw --;
if(nw >= 0) nw = last[0][nw];
else nw = -1000000;
if(nw != -1000000)
ans = min(ans, i - nw + 1 - 3 * k);
}
if(ans == 2000000000) ans = -1;
printf("%d\n", ans);
return 0;
}
