#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define F first
#define S second
#define pb push_back
#define pii pair<int, int>
pii a[505];
int n, m;
double res = 1e9;
bool cmp(pii x, pii y) {
if (x.F != y.F) return x.F < y.F;
return x.S < y.S;
}
double f[505];
void cal(int k) {
for (int i = 0; i <= n; i++) {
f[i] = 1e9;
}
f[0] = 0;
for (int i = 1; i <= n; i++) {
// cout << a[i].F << ' ' << a[i].S << endl;
for (int j = m - k; j >= 0; j--) {
if (j < m - k) {
f[j + 1] = min(f[j + 1], f[j] + (double)a[i].S / (k + 1));
}
int x = i - j;
if (0 < x && x <= k) f[j] += (double)a[i].F / x;
}
}
// cout << f[n][k] << endl;
// cout << f[m - k] << endl;
res = min(res, f[m - k]);
}
signed main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cin >> n >> m;
for (int i = 1; i <= n; i++) {
cin >> a[i].S >> a[i].F;
if (a[i].F == -1) a[i].F = 1e9;
}
sort(a + 1, a + 1 + n, cmp);
for (int i = 0; i < m; i++) {
cal(i);
}
cout << fixed << setprecision(20) << res;
}
