//#pragma GCC optimize("Ofast,O2,O3,unroll-loops")
//#pragma GCC target("avx2")
#include <bits/stdc++.h>
using namespace std;
void debug_out() { cerr << endl; }
template<typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
cerr << "[" << H << "]";
debug_out(T...);
}
#ifdef dddxxz
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif
#define SZ(s) ((int)s.size())
#define all(x) (x).begin(), (x).end()
#define lla(x) (x).rbegin(), (x).rend()
#define bpc(x) __builtin_popcount(x)
#define bpcll(x) __builtin_popcountll(x)
#define MP make_pair
clock_t startTime;
double getCurrentTime() {
return (double) (clock() - startTime) / CLOCKS_PER_SEC;
}
mt19937 rng(chrono::high_resolution_clock::now().time_since_epoch().count());
typedef long long ll;
const int MOD = 1e9 + 7;
const int INF = 1000000101;
const long long LLINF = 1223372000000000555;
const int N = 1e6 + 3e2;
const int M = 5555;
typedef long double ld;
void solve(int TC) {
int n, k;
cin >> n >> k;
vector<int> a(n), b(n);
for (int i = 0; i < n; i++) cin >> a[i] >> b[i];
ld ans = 1e18;
for (int mask = 0; mask < (1 << n); mask++){
vector<int> v;
bool ok = true;
for (int i = 0; i < n; i++){
if (mask & (1 << i)) {
if (b[i] == -1) ok = false;
v.push_back(b[i]);
}
}
if (!ok) continue;
int m = (int)v.size();
sort(all(v));
ld cur = 0;
for (int i = 0; i < m; i++){
cur += (ld)v[i] / (i + 1);
}
v.clear();
for (int i = 0; i < n; i++){
if (!(mask & (1 << i))) v.push_back(a[i]);
}
sort(all(v));
for (int i = 0; i < k - m; i++){
cur += (ld)v[i] / (m + 1);
}
ans = min(ans, cur);
}
cout << fixed << setprecision(15) << ans << endl;
}
int main() {
startTime = clock();
ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);
#ifdef dddxxz
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
#endif
int TC = 1;
// cin >> TC;
for (int test = 1; test <= TC; test++) {
//debug(test);
//cout << "Case #" << test << ": ";
solve(test);
}
#ifdef dddxxz
cerr << endl << "Time: " << int(getCurrentTime() * 1000) << " ms" << endl;
#endif
return 0;
}
