//Sylwia Sapkowska
#include <bits/stdc++.h>
#pragma GCC optimize("O3", "unroll-loops")
using namespace std;
void __print(int x) {cerr << x;}
void __print(long long x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << '\'' << x << '\'';}
void __print(const char *x) {cerr << '\"' << x << '\"';}
void __print(const string &x) {cerr << '\"' << x << '\"';}
void __print(bool x) {cerr << (x ? "true" : "false");}
template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ", "; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? ", " : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#ifdef LOCAL
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
#define debug(x...)
#endif
#define int long long
typedef pair<int, int> T;
typedef long double ld;
const int oo = 1e18, oo2 = 1e9+7, K = 30;
const int mod = 998244353;
void solve(){
int n, k; cin >> n >> k;
vector<T>a(n);
int power = 1;
for (int i = 1; i<=n; i++) power *= 3;
for (int i = 0; i<n; i++) {
cin >> a[i].first >> a[i].second;
if (a[i].second == -1) a[i].second = oo;
}
auto change = [&](int x){
string s;
while (x){
s += (char)((x%3) + '0');
x/=3;
}
reverse(s.begin(), s.end());
return s;
};
ld ans = oo;
vector<int>ord(n);
iota(ord.begin(), ord.end(), 0);
do{
vector<T>b(n);
for (int i = 0; i<n; i++) b[i] = a[ord[i]];
for (int mask = 1; mask < power; mask++){
string s = change(mask);
// debug(mask, s);
ld curr = 0;
int col = 1, vote = 0;
for (int i = 0; i<n; i++){
if (s[i] == '0'){
continue;
} else if (s[i] == '1'){
curr += (ld)(b[i].first)/(ld)(col);
vote++;
} else {
if (a[i].second == oo) break;
curr += (ld)(b[i].second)/(ld)(col);
col++;
vote++;
}
if (vote == k) {
ans = min(ans, curr);
break;
}
}
}
} while (next_permutation(ord.begin(), ord.end()));
cout << fixed << setprecision(15) << ans << "\n";
}
int32_t main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int t = 1;
// cin >> t;
while (t--) solve();
return 0;
}
