#include <iostream>
#include <fstream>
#include <algorithm>
#include <string>
#include <set>
#include <map>
#include <vector>
#include <math.h>
using namespace std;
typedef long long LL;
typedef vector<int> vi;
const int B = 10;
const int T = 107;
map<int, double> dp[1 << B][T];
struct S
{
double s;
double p;
int mask;
bool operator < (const S& rhs) const
{
return s * p > rhs.s * rhs.p;
}
};
vector<S> v;
int t;
double D(int mask, int pos, int k)
{
if (pos == v.size() || k == t)
return 0;
if (mask & v[pos].mask)
return D(mask, pos + 1, k);
double& res = dp[mask][pos][k];
if (res > 1)
return res - 2;
res = 2;
res += v[pos].p * (v[pos].s + D(mask | v[pos].mask, pos + 1, k + 1));
res += (1 - v[pos].p) * D(mask, pos + 1, k + 1);
return res - 2;
}
int main()
{
int b;
double f;
cin >> b >> t >> f;
for (int i = 0; i < b; ++i)
{
S s;
s.mask = 1 << i;
cin >> s.p >> s.s;
for (int j = 0; j < t; ++j)
{
v.push_back(s);
s.p *= f;
}
}
sort(v.begin(), v.end());
double res = D(0, 0, 0);
printf("%0.9lf\n", res);
return 0;
}
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