// ~~ icebear ~~
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> ii;
typedef pair<int, ii> iii;
template<class T>
bool minimize(T &a, const T &b) {
if (a > b) return a = b, true;
return false;
}
template<class T>
bool maximize(T &a, const T &b) {
if (a < b) return a = b, true;
return false;
}
#define FOR(i,a,b) for(int i=(a); i<=(b); ++i)
#define FORR(i,a,b) for(int i=(a); i>=(b); --i)
#define REP(i, n) for(int i=0; i<(n); ++i)
#define RED(i, n) for(int i=(n)-1; i>=0; --i)
#define MASK(i) (1LL << (i))
#define BIT(S, i) (((S) >> (i)) & 1)
#define mp make_pair
#define pb push_back
#define fi first
#define se second
#define all(x) x.begin(), x.end()
#define task "icebear"
const int MOD = 1e9 + 7;
const int inf = 1e9 + 27092008;
const ll INF = 1e18 + 27092008;
const int N = 2e5 + 5;
int n, k, f[N][3];
string S;
void init(void) {
cin >> n >> k >> S;
S = " " + S;
FOR(i, 1, n) {
f[i][0] = f[i - 1][0] + (S[i] == 'J');
f[i][1] = f[i - 1][1] + (S[i] == 'O');
f[i][2] = f[i - 1][2] + (S[i] == 'I');
}
}
void process(void) {
int l = 1, result = inf;
FOR(r, 1, n) {
while(l <= n && f[r][1] - f[l - 1][1] > k) l++;
while(f[r][1] - f[l - 1][1] == k && S[l] != 'O') l++;
if (f[r][1] - f[l - 1][1] == k && f[l - 1][0] >= k && f[n][2] - f[r][2] >= k) {
int low = 1, high = l - 1, res = 0, ans = 0;
while(low <= high) {
int mid = (low + high) >> 1;
if (f[l - 1][0] - f[mid - 1][0] >= k) res = mid, low = mid + 1;
else high = mid - 1;
}
low = r + 1, high = n;
while(low <= high) {
int mid = (low + high) >> 1;
if (f[mid][2] - f[r][2] >= k) ans = mid, high = mid - 1;
else low = mid + 1;
}
minimize(result, ans - res + 1 - 3 * k);
}
}
cout << (result == inf ? -1 : result);
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
if (fopen(task".inp", "r")) {
freopen(task".inp", "r", stdin);
freopen(task".out", "w", stdout);
}
int tc = 1;
// cin >> tc;
while(tc--) {
init();
process();
}
return 0;
}
