#include <bits/stdc++.h>
using namespace std;
#define int long long
#define rep(i,a,b) for (int i = a; i <= b; ++i)
#define pii pair<int,int>
#define fi first
#define se second
#define pb push_back
#define all(v) (v).begin(), (v).end()
const int MAXN = 510;
pii a[MAXN];
bool mk[MAXN];
int32_t main() {
ios_base::sync_with_stdio(0); cin.tie(nullptr);
int n,k; cin >> n >> k;
rep(i,1,n) cin >> a[i].fi >> a[i].se;
double ans = 1e9;
do {
rep(j,0,k) {
double cur = 0;
int cnt = 1;
bool ok = 1;
rep(i,1,j) {
if (a[i].se == -1) {
ok = 0;
break;
}
cur += (double)a[i].se/cnt;
cnt++;
}
if (!ok) break;
rep(i,j+1,k) {
cur += (double)a[i].fi/cnt;
}
ans = min(cur,ans);
}
}while(next_permutation(a+1,a+n+1));
cout << ans << '\n';
}
