#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
constexpr int MAXN = 5e2 + 7;
constexpr ld oo = 1e9;
tuple<ld, ld, int> T[MAXN];
pair<ld, ld> TORG[MAXN];
pair<int, int> prevs[MAXN][MAXN];
ld dp[MAXN][MAXN];
bool blocked[MAXN];
ld sumA;
int n, k;
bool cmp1(tuple<ld, ld, int> a, tuple<ld, ld, int> b){
auto [xa, ya, ia] = a;
auto [xb, yb, ib] = b;
if(ya != yb)
return ya < yb;
return xa < xb;
}
bool cmp2(tuple<ld, ld, int> a, tuple<ld, ld, int> b){
auto [xa, ya, ia] = a;
auto [xb, yb, ib] = b;
if(xa != xb)
return xa < xb;
return ya < yb;
}
ld compute(ld val, ld l){
return (sumA + val)/l;
}
void compute(){
for(int i = 1; i <= n; i++){
dp[i][1] = std::get<1>(T[i]) - std::get<0>(T[i]);
prevs[i][1] = {0, std::get<2>(T[i])};
}
for(int l = 2; l <= k; l++){
for(int i = l; i <= n; i++){
ld actbest = (oo*oo);
for(int j = l-1; j < i; j++){
if(actbest > compute(dp[j][l-1] + dp[i][1], l)){
prevs[i][l] = {j, std::get<2>(T[i])};
dp[i][l] = dp[j][l-1] + dp[i][1];
actbest = compute(dp[i][l], l);
}
}
}
}
}
vector<int> odtworz(int i, int l){
vector<int> out;
int pocz = i;
while(l){
out.push_back(prevs[pocz][l].second);
//cout << "k: " << l << " " << out.back() << "\n";
pocz = prevs[pocz][l--].first;
}
reverse(out.begin(), out.end());
return out;
}
ld solve(){
ld res = oo;
sort(T+1, T+1+n, cmp2);
for(int l = 0; l <= k-1; l++){
for(int i = l; i <= n; i++){
vector<int> odt = odtworz(i, l);
ld act = 0;
for(int z = 1; z <= l; z++){
act += (TORG[odt[z-1]].second/z);
blocked[odt[z-1]] = 1;
}
int it = k - l;
for(int w = 1; w <= n && it > 0; w++){
auto [xa, ya, ia] = T[w];
if(blocked[ia]) continue;
act += (xa / (l+1));
it--;
}
for(auto u : odt)
blocked[u] = 0;
//cout << "k: " << l << " " << act << "\n";
res = min(res, act);
}
}
return res;
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cin >> n >> k;
for(int i = 1; i <= n; i++){
ld a, b;
cin >> a >> b;
sumA += a;
if(b == -1) b = 1e9;
TORG[i] = {a, b};
T[i] = {a, b, i};
}
sort(T+1, T+1+n, cmp1);
compute();
cout << fixed << setprecision(9) << solve() << "\n";
}
