#include<bits/stdc++.h>
#define taskname "c"
using namespace std;
const int lim = 505;
template<class T>void minimize(T& a, T b){
if(a > b){
a = b;
}
}
int n, k, a[lim], b[lim];
namespace sub12{
void solve(){
vector<int>p;
for(int i = 1; i <= n; i++){
if(b[i] != -1){
p.push_back(i);
}
}
sort(p.begin(), p.end(), [&] (int i, int j){
return b[i] < b[j];
});
double ans = 0;
vector<int>I(n);
iota(I.begin(), I.end(), 1);
sort(I.begin(), I.end(), [&] (int i, int j){
return a[i] < a[j];
});
for(int i = 0; i < k; i++){
ans += a[I[i]];
}
vector<bool>vis(n + 1, false);
double sum = 0;
for(int i = 0; i < min(int(p.size()), k); i++){
vis[p[i]] = true;
sum += double(b[p[i]]) / double(i + 1);
double current = 0;
int cnt = i + 1;
for(int& j : I){
if(cnt == k){
break;
}
if(!vis[j]){
current += double(a[j]) / double(i + 2);
cnt++;
}
}
if(cnt == k){
minimize(ans, current + sum);
}
}
cout << setprecision(9) << fixed << ans;
}
}
namespace sub34{
void solve(){
for(int i = 0; i < n; i++){
a[i] = a[i + 1];
b[i] = b[i + 1];
}
double ans = 1e18;
for(int mask = 0; mask < (1 << n); mask++){
bool f = true;
double current = 0;
vector<int>A, B;
for(int i = 0; i < n; i++){
if(mask >> i & 1){
A.push_back(a[i]);
current += a[i];
}
else if(b[i] != -1){
B.push_back(b[i]);
}
}
if(__builtin_popcount(mask) == k){
minimize(ans, current);
}
sort(A.begin(), A.end());
sort(B.begin(), B.end());
double sum = 0;
for(int i = 0; i < min(int(B.size()), k); i++){
sum += double(B[i]) / double(i + 1);
if(k - i - 1 <= A.size()){
for(int j = current = 0; j < k - i - 1; j++){
current += double(A[j]) / double(i + 2);
}
minimize(ans, sum + current);
}
}
}
cout << setprecision(9) << fixed << ans;
}
}
namespace sub5{
void solve(){
vector<int>p(n);
iota(p.begin(), p.end(), 1);
sort(p.begin(), p.end(), [&] (int i, int j){
return a[i] < a[j];
});
double ans = 0;
for(int i = 0; i < k; i++){
ans += a[p[i]];
}
sort(p.begin(), p.end(), [&] (int i, int j){
return b[i] < b[j];
});
for(int i = 1; i < k; i++){
vector<vector<double>>dp(i + 1, vector<double>(k - i + 1, 1e18));
for(int j = dp[0][0] = 0; j < n; j++){
for(int x = i; x > -1; x--){
for(int y = k - i; y > -1; y--){
if(b[p[j]] != -1 && x > 0){
minimize(dp[x][y], dp[x - 1][y] + double(b[p[j]]) / double(x));
}
if(y > 0){
minimize(dp[x][y], dp[x][y - 1] + double(a[p[j]]) / double(i + 1));
}
}
}
}
minimize(ans, dp[i][k - i]);
}
cout << setprecision(9) << fixed << ans;
}
}
namespace sub67{
void solve(){
vector<int>p(n);
iota(p.begin(), p.end(), 1);
for(int i = 1; i <= n; i++){
if(b[i] == -1){
b[i] = 2e9;
}
}
sort(p.begin(), p.end(), [&] (int i, int j){
return b[i] < b[j] || (b[i] == b[j] && a[i] < a[j]);
});
double ans = 1e18;
for(int i = 0; i < k; i++){
vector<double>dp(n + 1, 1e18);
for(int x = dp[0] = 0; x <= n; x++){
vector<double>ndp(n + 1, 1e18);
for(int y = 0; y <= x; y++){
int yy = min(i, x - y);
if(yy + y == k && yy == i){
minimize(ans, dp[y]);
}
if(x < n){
minimize(ndp[y + 1], dp[y] + double(a[p[x]]) / double(i + 1));
minimize(ndp[y], dp[y] + (yy == i ? 0 : double(b[p[x]]) / double(yy + 1)));
}
}
swap(dp, ndp);
}
}
cout << setprecision(9) << fixed << ans;
}
}
int main(){
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
if(fopen(taskname".inp", "r")){
freopen(taskname".inp", "r", stdin);
}
cin >> n >> k;
bool is_sub12 = true;
for(int i = 1; i <= n; i++){
cin >> a[i] >> b[i];
if(b[i] != -1 && a[i] != b[i]){
is_sub12 = false;
}
}
if(is_sub12){
sub12::solve();
}
else if(n <= 20){
sub34::solve();
}
else if(n <= 100){
sub5::solve();
}
else{
sub67::solve();
}
}
