#include <bits/stdc++.h>
#define fastIO ios_base::sync_with_stdio(false); cin.tie(NULL)
#define file(a) freopen(a".inp","r",stdin); freopen(a".out", "w",stdout)
#define fi first
#define se second
#define endl "\n"
#define len(s) (int)((s).size())
#define MASK(k)(1LL<<(k))
#define TASK "test"
#define int long long
using namespace std;
typedef tuple<int, int, int> tpiii;
typedef pair<double, double> pdd;
typedef pair<int, int> pii;
typedef long long ll;
const long long OO = 1e18;
const int MOD = 1e9 + 7;
const int INF = 1e9;
const int MX = 505;
int n, k;
pii a[MX];
double f[2][505];
int32_t main(){
fastIO;
//file(TASK);
cin >> n >> k;
for(int i=1; i<=n; i++){
cin >> a[i].se >> a[i].fi;
if(a[i].fi == -1) a[i].fi = OO;
}
sort(a + 1, a + 1 + n);
double ans = OO;
for(int t=0; t<=k; t++) if(a[t].fi < OO){
for(int i=0; i<MX; i++) f[0][i] = f[1][i] = OO;
f[0][0] = 0;
for(int i=1; i<=n; i++){
int c = i&1;
for(int j=0; j<=min(i, k-t); j++){
f[c][j] = f[c^1][j];
if(i - j <= t) f[c][j] += 1.0*a[i].fi/(i - j);
if(j) f[c][j] = min(f[c][j], f[c^1][j-1] + 1.0*a[i].se/(t + 1));
}
}
ans = min(ans, f[n&1][k - t]);
}
cout << setprecision(9) << fixed << ans;
return 0;
}
