这段C语言程序哪里有错？

#include <stdio.h>
#include <math.h>
int mul(int n);
int main()
{   
   
    FILE *fp;
    if ((fp=fopen("SC3_6C.in","r"))!=NULL)
        {freopen("SC3_6C.in", "r", stdin);
    freopen("SC3_6C.out", "w", stdout);}
    int i,j,a,n;
    double sn,sum=0;
   //************************************
scanf("%d%d",&a,&n);
j=n;
for(i=1;i<=n;i++)
{ sum=sum+j*a*(mul(i));
   j--;
}
printf("%.f",sum);
 //====================================
    return 0;
}
int mul(int n)
{
int i,sn=1;
for(i=2;i<=n;i++)
{
sn=sn*10;
}
return sn;
}