text: #include <bits/stdc++.h>
  
  using namespace std;
  
  int main(){
      map<int, double> t;
      for(int i = 1; i < 6; ++i)
          t[1000+i] = i + 0.5;
  
      int n, x, p, q;
      double soma = 0;
  
      scanf("%d", &n);
      while(n--)
      {
          scanf("%d %d", &p, &q);
          soma += t[p] * q;
      }
      printf("%.2lf\n", soma);
      return 0;
  }
  