#include <stdio.h>
int main()
{
    int kotobar;
    float price;
    int code, koyta;
    scanf("%d", &kotobar;);
    while (kotobar--)
    {
        scanf("%d %d", &code;, &koyta;);
        if (code == 1001)
            price += koyta * 1.50;
        else if (code == 1002)
            price += koyta * 2.50;
        else if (code == 1003)
            price += koyta * 3.50;
        else if (code == 1004)
            price += koyta * 4.50;
        else if (code == 1005)
            price += koyta * 5.50;
    }
    printf("%.2f\n", price);
    return 0;
}