#include <bits/stdc++.h>
#pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4.1,sse4.2,avx,avx2,popcnt,tune=native")
#define jizz ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define ll long long
using namespace std;
char str[200005];
ll s,t;
ll n, k;
ll ans = 200005;
ll loc, cnt;
vector <ll> locJ;
vector <ll> locO;
vector <ll> locI;
int main() {
scanf("%lld%lld", &n, &k);
scanf("%s", str);
s = 0;
//枚舉J，greedyO跟I (upper_bound優化
for(ll i = 0; i < n; i++){
if(str[i]=='J')locJ.push_back(i);
if(str[i]=='O')locO.push_back(i);
if(str[i]=='I')locI.push_back(i);
}
for(ll i = 0; i < locJ.size()-k+1; i++){
ll num = 0;
bool ok = 1;
num+=locJ[i+k-1]-locJ[i]+1-k;
// printf("J%lld %lld\n", locJ[i+k-1], locJ[i]);
s = upper_bound(locO.begin(), locO.end(), locJ[i+k-1])-locO.begin();
if(s<0) ok = 0;
for (ll j = 1; j < k; j++){
s = upper_bound(locO.begin(), locO.end(), locO[s])-locO.begin();
if(s<0||!ok){
ok = 0;
break;
}
}
if(locO[s]<locJ[i+k-1]) ok = 0;
if(ok){
num+=locO[s]-locJ[i+k-1]-k;
// printf("O%lld %lld\n", locO[s], locJ[i+k-1]);
t = upper_bound(locI.begin(), locI.end(), locO[s])-locI.begin();
if(t<0) ok = 0;
for (ll j = 1; j<k; j++){
t = upper_bound(locI.begin(), locI.end(), locI[t])-locI.begin();
if(t<0||!ok) {
ok = 0;
break;
}
}
if(locI[t]<locO[s])ok = 0;
if(ok) {
num+=locI[t]-locO[s]-k;
ans = min(num, ans);
// printf("I%lld %lld\n", locI[t], locO[s]);
}
}
// printf("%d ", ok);
// if(ok)
// printf("%lld\n", num);
// else
// printf("-1\n");
}
if(ans<200000)
printf("%lld", ans);
else
printf("-1");
return 0;
}
