#include<stdio.h>
#define maxsize 100
int main(void)
{
    double x[maxsize],y[maxsize],aa,sum,p,a,b;
    int i,k,n;

    printf("please input a and b:\n");
    scanf("%lf %lf",&a,&b);

    printf("please input aa=\n");
    scanf("%lf",&aa);

    printf("please input n=\n");
    scanf("%d",&n);

    printf("please input x[k]=\n");
    for(k=0;k<=n;k++)
    {
        printf("x[%d]=",k);
        scanf("%lf",&x[k]);
    }

    printf("please input y[k]=\n");
    for(k=0;k<=n;k++)
    {
        printf("y[%d]=",k);
        scanf("%lf",&y[k]);
    }

    if(x[0]<a&&x[n]>b)
    {
        printf("输入错误，请重新输入!");
        return 0;
    }
    sum=0;               
    for(k=0;k<=n;k++)
    {
        p=1;
        for(i=0;i<=n;i++)
        
            if(i!=k)
            
                p=p*(aa-x[i])/(x[k]-x[i]);
                    sum+=p*y[k];
            
        
    }
    printf("sum=%lf\n",sum);
    return 0;
}