#include <bits/stdc++.h>
using namespace std;
#define foreach(a,b,i) for(type(a) i = a; i <= b; i++)
#define type(x) __typeof(x)
#define pb(x) push_back(x)
#define mp(x,y) make_pair(x,y)
int N,K;
vector<pair<long double,int>> v;
int A[510];
long double B[510];
long double pre[510][510];
const long double inf = 1e20;
long double memo[510][510];
int main(){
scanf(" %d",&N);
scanf(" %d",&K);
for(int i = 1; i <= N; i++){
scanf(" %d",&A[i]);
int h;
scanf(" %d",&h);
if(h == -1) B[i] = inf;
else B[i] = h;
v.pb(mp(B[i],i));
}
v.pb(mp(-1,-1));
sort(v.begin(),v.end());
foreach(0,N - 1,i){
pre[i][i] = 0;
foreach(i + 1, N, j){
pre[j][i] = pre[j - 1][i] + B[v[j].second] / (long double) (j - i);
}
}
long double big = inf;
foreach(0,K,i){
foreach(0,K,k){
memo[0][k] = inf;
}
memo[0][0] = pre[i][0] - pre[0][0];
//printf("%Lf\n",memo[0][0]);
foreach(1,N, j){
foreach(0, K - i, k){
memo[j][k] = inf;
if(k == 0){
memo[j][k] = memo[j - 1][k];
}
else{
if(j <= i + k - 1){
memo[j][k] = min(memo[j - 1][k], memo[j - 1][k - 1] + A[v[j].second]/(long double)(i+1) + pre[i + k][k] - pre[j][k] - (pre[i + k - 1][k - 1] - pre[j - 1][k - 1]) );
}
else{
memo[j][k] = min(memo[j - 1][k], memo[j - 1][k - 1] + A[v[j].second]/(long double)(i+1));
}
}
//printf("%Lf %d %d\n",memo[j][k],j,k);
}
}
//printf("\n%Lf %d\n\n",memo[N][K - i],i);
big = min(big,memo[N][K - i]);
}
printf("%Lf",big);
}
/*
7
7
4 -1
11 -1
6 -1
12 -1
36 -1
11 -1
20 -1
*/
