#include <algorithm>
#include <iostream>
#include <vector>

using namespace std;
using ll = long long;
using db = double;
using PDL = pair<db, ll>;

int main() {
    db ans = 0;
    int n, k, s, cnt = 0;
    vector<PDL> p(n * 2);

    cin >> n >> k >> s;
    for (int i = 1; i <= n; i++) {
        int x, y;
        cin >> x >> y;  // num value
        if (x > k) {
            p.push_back({ y, k });
            p.push_back({ y / 2.0, x - k });
        } else
            p.push_back({ y, x });
    }

    sort(p.begin(), p.end(), greater<PDL>());

    for (auto t : p) {
        double v = t.first;
        ll m = t.second;
        if (cnt + m < s)
            ans += m * v, cnt += m;
        else {
            ans += (s - cnt) * v;
            break;
        }
        // else if (cnt + m >= s && cnt < s)
        // ans += (s - cnt) * v, cnt = s;
    }
    printf("%.1lf\n", ans);
}