#include <bits/stdc++.h>
#define int long long
#define FOR(i, a, b) for (int i = (a), _b = (b); i <= _b; ++i)
#define REV(i, a, b) for (int i = (a), _b = (b); i >= _b; --i)
using namespace std;
#define MAXN 200005
int n, k;
string s;
int prefj[MAXN];
int suffi[MAXN];
int prefo[MAXN];
const int inf = 1e16;
int pos[MAXN];
int prefmi[MAXN];
void solve(){
cin >> n >> k;
cin >> s;
int ptr = 1;
int sum = 0;
int cnt = 0;
FOR(i, 0, n + 1) prefj[i] = inf;
FOR(i, 0, n + 1) suffi[i] = inf;
FOR(i, 1, n){
if (s[i - 1] == 'J') sum++;
else cnt++;
while (sum > k || s[ptr - 1] != 'J'){
if (s[ptr - 1] == 'J') sum--;
else cnt--;
ptr++;
}
if (sum == k){
prefj[i] = cnt;
}
}
ptr = n;
sum = 0;
cnt = 0;
REV(i, n, 1){
if (s[i - 1] == 'I') sum++;
else cnt++;
while (sum > k || s[ptr - 1] != 'I') {
if (s[ptr - 1] == 'I') sum--;
else cnt--;
ptr--;
}
if (sum == k) {
suffi[i] = cnt;
}
}
FOR(i, 1, n) {
prefo[i] = prefo[i - 1];
if (s[i - 1] == 'O') prefo[i]++;
pos[prefo[i]] = i;
}
FOR(i, 0, n + 1) prefmi[i] = inf;
FOR(i, 1, n){
prefmi[i] = prefmi[i - 1];
prefmi[i] = min(prefmi[i], - i - k + prefj[i]);
}
int res = inf;
FOR(i, 1, n){
if (prefo[i] < k) continue;
int need = pos[prefo[i] - k];
res = min(res, suffi[i + 1] + prefmi[need] + i);
}
if (res > inf / 10) res = -1;
cout << res;
}
int32_t main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr); cout.tie(nullptr);
solve();
return 0;
}
