#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ld long double
#define pii pair<ll, ll>
#define fi first
#define sec second
const int MAXN = 5e2;
const ll INF = 2e18;
ll a[MAXN + 5], b[MAXN + 5];
ld dp[2][MAXN + 5][MAXN + 5];
ll sum[MAXN + 5];
int main(){
ll N, K; cin >> N >> K;
vector<pii> v(1);
for(int i = 1; i <= N; i++){
cin >> a[i] >> b[i];
if(b[i] == -1) b[i] = INF;
v.push_back({b[i], a[i]});
}
sort(v.begin(), v.end());
for(int i = 0; i < 2; i++){
for(int j = 0; j <= N; j++){
for(int l = 0; l <= N; l++) dp[i][j][l] = INF;
}
}
vector<ll> tmp;
for(int i = N; i >= 1; --i){
tmp.push_back(v[i].sec);
sort(tmp.begin(), tmp.end());
if(K - (i - 1) >= (int)tmp.size()){
for(int k = 0; k < K - (i - 1); k++) sum[i] += tmp[k];
}
else sum[i] = INF;
}
ld ans = INF;
ans = min(ans, (ld)sum[1]);
for(int i = 0; i <= K; i++) dp[0][0][i] = 0;
for(int i = 1; i <= K; i++){
for(int k = 0; k <= i; k++){
for(int l = 0; l <= K; l++){
dp[i % 2][k][l] = INF;
// take buff
if(k && v[i].fi != INF) dp[i % 2][k][l] = min(dp[i % 2][k][l], dp[(i % 2) ^ 1][k - 1][l] + (1 / (1.0 * k)) * (1.0 * v[i].fi));
// not take buff
dp[i % 2][k][l] = min(dp[i % 2][k][l], dp[(i % 2) ^ 1][k][l] + (1 / (1.0 * (l + 1))) * (1.0 * v[i].sec));
}
}
for(int j = 0; j <= i; j++){
ans = min(ans, sum[i + 1] * (1 / (1.0 * (j + 1))) + dp[i % 2][j][j]);
}
}
cout << fixed << setprecision(10) << ans << "\n";
}
