#include <bits/stdc++.h>
#pragma GCC optimize("O3", "Ofast")
#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 = 5e5+10;
const int MAXA = 5e5+10;
const int SQRT = 450;
const ld INF = 1e16;
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];
ld dp[510][510][510];
signed main(){
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
cin>>n>>need;
vector<pii> vec; vec.pb({-1,-1});
for(int i=1; i<=n; i++){
cin>>a[i]>>b[i];
if(b[i]==-1) b[i] = INF;
vec.pb({b[i], a[i]});
}
sort(vec.begin(), vec.end());
for(int i=1; i<=n; i++)
a[i] = vec[i].se, b[i] = vec[i].fi;
for(int i=0; i<=n; i++)
for(int j=0; j<=n; j++)
for(int k=0; k<=need; k++) dp[i][j][k] = INF;
dp[0][0][0] = 0;
for(int i=1; i<=n; i++){
// cout << a[i] << ' ' << b[i] << " b\n";
for(int j=0; j<=need; j++){ // ambil speed nya
for(int k=j; k<=need; k++){
dp[i][j][k] = dp[i-1][j][k];
if(k!=0 && j!=0 && b[i]!=INF) // ambil b
chmn(dp[i][j][k], dp[i-1][j-1][k-1] + (ld)b[i]/j);
if(k!=0) // ambil a
chmn(dp[i][j][k], dp[i-1][j][k-1] + (ld)a[i]/(j+1) );
}
}
// cout << dp[2][1][2] << " pp\n\n";
}
ld ANS = INF;
for(int j=0; j<=n; j++)
chmn(ANS, dp[n][j][need]);
cout << fixed << setprecision(5) << ANS << '\n';
}
