/*
ID: y1197771
PROG: test
LANG: C++
*/
#include<bits/stdc++.h>
#define pb push_back
#define FOR(i, n) for (int i = 0; i < (int)n; ++i)
#define dbg(x) cout << #x << " at line " << __LINE__ << " is: " << x << endl
typedef long long ll;
using namespace std;
typedef pair<int, int> pii;
const int maxn = 1e3 + 10;
void solve() {
    int t, n, w;
    double s = 0;
    cin >> t >> n >> w;
    double task; int p;
    for (int i = 0; i < t; i++) {
        cin >> task >> p;
        s += task * (100 - p) / 100;
    }
    printf("%.2f\n", s / w);
}
int main() {
    //freopen("test.in", "r", stdin);
    //freopen("test.out", "w", stdout);
    solve();
    return 0;
}
 

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

NiAzIDkwCjEwMCA5MAo0MC40MCA3MAo2MC4zMCA3MAo0MC40MCA4MAo0MC40MCA4NQo0MC40MCA4OAo=

6 3 90
100 90
40.40 70
60.30 70
40.40 80
40.40 85
40.40 88