#include <bits/stdc++.h>
#define pb push_back
#define f first
#define sc second
using namespace std;
typedef long long int ll;
typedef string str;
typedef double ld;
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cerr.precision(100);
cout.precision(100);
int n, k; cin >> n >> k;
vector<pair<int, int>> v(n);
for(auto &p: v) cin >> p.f >> p.sc;
sort(v.begin(), v.end(), [](pair<ld, ld> a, pair<ld, ld> b){
swap(a.f, a.sc);
swap(b.f, b.sc);
if(a.f == -1) a.f = 1e9;
if(b.f == -1) b.f = 1e9;
return a < b;
});
ld ans = 1e9;
vector<vector<vector<ld>>> dp;
for(int c = 0; c <= k; c++){
dp = vector<vector<vector<ld>>>(n, vector<vector<ld>>(c+1, vector<ld>(k-c+1, 1e6)));
for(int i = 0; i < n; i++) dp[i][0][0] = 0;
for(int i = n-1; i >= 0; i--){
for(int j0 = 0; j0 <= c; j0++) for(int j1 = 0; j1 <= k-c && j0+j1 <= n-i; j1++){
if(j1+j0 == 0) continue;
if(i == n-1){
if(j1 == 1) dp[i][j0][j1] = (ld)v[i].f/(ld)(c+1);
else if(j0 == 1 && v[i].sc != -1) dp[i][j0][j1] = (ld)v[i].sc/(ld)c;
continue;
}
dp[i][j0][j1] = dp[i+1][j0][j1];
if(j1 != 0) dp[i][j0][j1] = min(dp[i][j0][j1], (ld)v[i].f/(ld)(c+1) + dp[i+1][j0][j1-1]);
if(v[i].sc != -1 && j0 != 0){
dp[i][j0][j1] = min(dp[i][j0][j1], (ld)v[i].sc/(ld)(c-j0+1) + dp[i+1][j0-1][j1]);
}
}
}
ans = min(ans, dp[0][c][k-c]);
}
cout << ans << "\n";
}
