#include<stdio.h>
int main(){
int n,a; double b;
scanf("%d",&n);
double i; double j=0; double k=0;
for(i=0;i<n;i++){
scanf("%d %lf",&a,&b);
j+=a*b;
k+=a;
}
printf("%.2f",j/k);return 0;
}
#include<stdio.h>
int main(){
int n,a; double b;
scanf("%d",&n);
double i; double j=0; double k=0;
for(i=0;i<n;i++){
scanf("%d %lf",&a,&b);
j+=a*b;
k+=a;
}
printf("%.2f",j/k);return 0;
}