#include <iostream>
#include <cmath>
using namespace std;
int main()
{
int member1, sum2 = 0;
double member2, res = 0, sum1 = 0;
while (cin >> member1 >> member2)
{
if (member1=='e') break;
sum1 += member1 * member2;
sum2 += member1;
}
res = sum1 / sum2;
cout<< res << endl;
return 0;
}
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