#include <stdio.h>
int main()
{
    int qtd;
    double pr,gas_temp,gas;
    do{
        scanf("%d",&qtd);
        scanf("%lf",&pr);
        while(pr<0){
          scanf("%lf",&pr);
        }

        gas_temp=pr*qtd;
        gas = gas + gas_temp;
    }while(qtd>0);
    printf("%.2lf", gas);



    return 0;
}