hdu 1202 The calculation of GPA

#include <stdio.h>
int f(double x)
{
    if(x>=90)return 4;
    else if(x>=80)return 3;
    else if(x>=70)return 2;
    else if(x>=60)return 1;
    else return 0;
}

int main(void)
{
    int n;double cj,zjd,xf,zxf;
    while(scanf("%d",&n)!=EOF)
    {
        zxf=zjd=0;
        while(n--)
        {
            scanf("%lf%lf",&xf,&cj);
            if(cj==-1)continue;
            zxf+=xf;
            zjd+=xf*f(cj);
        }
        if(zjd>0)
            printf("%.2f\n",zjd/zxf);
        else
            printf("-1\n");
    }
}