#include <bits/stdc++.h>
#pragma GCC optimize("O3")
#define int long long
#define ll long long
#define se second
#define fi first
#define pb push_back
#define lf (id<<1)
#define rg ((id<<1)|1)
#define md ((l+r)>>1)
#define ld long double
using namespace std;
typedef pair<int,int> pii;
typedef pair<pii,pii> ipii;
const int MAXN = 510;
const int MAXA = 5e5+10;
const int SQRT = 450;
const ld INF = 1e16;
const int IN = 1e10;
const int MOD = 1e9+7;
const int LOG = 60;
int sum(int a, int b){ return (a+MOD+b)%MOD; }
void chsum(int &a, int b){ a = sum(a,b); }
int mul(int a, int b){ return (a*b)%MOD; }
void chmul(int &a, int b){ a = mul(a,b); }
void chmn(auto &a, auto b){ a = min(a, b); }
void chmx(auto &a, auto b){ a = max(a, b); }
vector<int> dx = {0, 0, -1, 1};
vector<int> dy = {1, -1, 0, 0};
int n, need, a[MAXN], b[MAXN], su[510][510], blm[MAXN], cost[MAXN];
ld dp[510][510];
ld ANS = INF;
void solve(int sp){
// sort based on b
// dari belakang, tambahin b
for(int i=0; i<=n+1; i++)
for(int j=0; j<=n+1; j++)
dp[i][j] = INF, su[i][j] = IN;
dp[0][0] = 0;
for(int i=1; i<=need; i++){
for(int j=0; j<=sp; j++){ // b yg udh diambil segini
// ambil b, ambil a
dp[i][j] = dp[i-1][j] + (ld)blm[i]/(sp+1);
if(j!=0 && cost[i]!=INF){ // ambil
chmn(dp[i][j], dp[i-1][j-1] + (ld)cost[i]/j);
}
}
}
su[n+1][0] = 0;
for(int j=0; j<=n; j++){
for(int i=n; i>=1; i--){
su[i][j] = su[i+1][j];
if(j!=0) chmn(su[i][j], su[i+1][j-1]+blm[i]);
}
}
// cout << fixed << setprecision(5) << dp[1][1] << "Xx\n";
// cout << ' ' << su[2][need-1] << " pp\n";
for(int i=sp; i<=need; i++){
chmn(ANS, dp[i][sp] + (ld)su[i+1][need-i] / (sp+1) );
}
}
signed main(){
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
cin>>n>>need;
vector<pii> cc; cc.pb({-1, -1});
for(int i=1; i<=n; i++){
cin>>a[i]>>b[i];
if(b[i] == -1) b[i] = INF;
cc.pb({b[i], a[i]});
}
sort(cc.begin(), cc.end());
for(int i=1; i<=n; i++)
cost[i] = cc[i].fi, blm[i] = cc[i].se;
for(int sp=0; sp<=need; sp++)
solve(sp);
cout << fixed << setprecision(5) << ANS << '\n';
}
