C语言求阶乘1!+3!+6!+10!+15!+21!急!!!!!!拜托了

输入7即是答案。

#include<stdio.h>

int mul(int arr[],int n,int len)
{
int i,c=0;
for (i=0; i<len; i++)
{
arr[i]=arr[i]*n+c;
c=arr[i]/10;
arr[i]%=10;
}
while (c!=0)
{
arr[i++]=c%10;
c/=10;
}
return i;
}

int add(int a[],int n,int b[],int m)
{
int i,c=0;
for (i=0; i<m; i++)
{
a[i]+=b[i]+c;
c=a[i]/10;
a[i]%=10;
}
while (c!=0)
{
a[i]+=c;
c=a[i]/10;
a[i]%=10;
i++;
}
return i;
}

int main()
{
int i,j,k,n,a_len,b_len,a[1024],b[1024];
while (scanf("%d",&n))
{
a_len=0;
for (i=0; i<1024; i++) a[i]=0;
for (i=1; i<=n; i++)
{