#include <bits/stdc++.h>
#define ios ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0)
#define file(s) if (fopen(s".in", "r")) freopen(s".in", "r", stdin), freopen(s".out", "w", stdout)
#define all(a) a.begin() , a.end()
#define F first
#define S second
using namespace std;
using ll = long long;
const int N = 500+5 , inf = 2e9 + 7;
const ll INF = 1e18 , mod = 1e9+7 , P = 6547;
int a[N], b[N];
double dp[N][N][N];
void mini(double &a, double b) {
a = min(a, b);
}
void solve(int tc) {
int n , k;
cin >> n >> k;
vector<pair<int,int>> vec;
for(int i = 1; i <= n; i ++) {
cin >> a[i] >> b[i];
if(b[i] == -1) b[i] = inf;
vec.push_back({b[i] , a[i]});
}
sort(all(vec));
for(int i = 1; i <= n; i ++) {
a[i] = vec[i-1].S;
b[i] = vec[i-1].F;
}
for(int i = 0; i <= n; i ++) {
for(int j = 0; j <= n; j ++) {
for(int k = 0; k <= n; k ++) {
dp[i][j][k] = INF;
}
}
}
for(int i = 0; i <= n; i ++) dp[i][0][0] = 0;
double ans = INF;
for(int l = 0; l <= n; l ++) { // в конце сторонников
for(int i = 0; i < n; i ++) {
for(int j = 0; j <= i; j ++) {
mini(dp[i+1][j][l], dp[i][j][l]);
mini(dp[i+1][j+1][l], dp[i][j][l] + a[i+1]/double(l+1));
if(i == j)
for(int x = 0; x <= min(j, l); x ++) {
mini(dp[i+1][j][x], dp[i][j][x]);
mini(dp[i+1][j+1][x], dp[i][j][x] + a[i+1]/double(l+1));
if(b[i+1] != inf)
mini(dp[i+1][j+1][x+1], dp[i][j][x] + b[i+1]/double(x+1));
}
}
}
ans = min(ans, dp[n][k][l]);
}
cout << fixed << setprecision(6) << ans;
}
/*
*/
main() {
ios;
int tt = 1 , tc = 0;
// cin >> tt;
while(tt --) {
solve(++tc);
}
return 0;
}
