#include <bits/stdc++.h>
using namespace std;
#define int long long
#define N 500005
const int inf = 1e18;
int k, n, a[N], p[N][5], ans = inf;
string s, g = "JOI";
int32_t main() {
ios::sync_with_stdio(0);cin.tie(0);
cin >> n >> k >> s;
int inx = 0;
map <char,int> vis;
for(int j = 0; j < 3; j++) {
vis[g[j]] = j + 1;
}
for(int i = 0; i < n; i++) {
a[i + 1] = vis[s[i]];
}
for(int i = 1; i <= n; i++) {
p[i][a[i]]++;
for(int j = 1; j <= 3; j++) {
p[i][j] += p[i - 1][j];
}
}
int b = 1, O = 0, J = 0, I = 0;
for(int i = 1; i <= n; i++) {
O += (a[i] == 2);
while(b <= i && O - (a[b] == 2) >= k) {
O -= (a[b] == 2);
b++;
}
int l = 1, r = b - 1, x1 = 0, x2 = 0;
while(l <= r) {
int mid = (l + r) / 2;
if(p[b - 1][1] - p[mid - 1][1] >= k) {
l = mid + 1;
x1 = mid;
}
else r = mid - 1;
}
l = i + 1, r = n;
while(l <= r) {
int mid = (l + r) / 2;
if(p[mid][3] - p[i][3] >= k) {
r = mid - 1;
x2 = mid;
}
else l = mid + 1;
}
if(!x1 || !x2) continue;
ans = min(ans, ((b - 1) - x1 + 1) + (i - b + 1) + (x2 - (i + 1) + 1) - (3 * k));
}
// cout << ans << '\n';
cout << (ans == inf ? -1 : ans) << '\n';
return 0;
}
