#include <iostream>
#include <vector>
using namespace std;
int main() {
vector<int> n;
vector<double> x;
int a, sum2 = 0;
double b, res = 0, sum1 = 0;
while (cin >> a >> b)
{
n.push_back(a);
x.push_back(b);
}
for (int i=0; i < x.size(); i++)
{
sum1 += n[i] * x[i];
sum2 += n[i];
res = sum1 / sum2;
}
cout << res << endl;
return 0;
}
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