poj 1354 Placement of Keys

#include <stdio.h>#include <string.h>#define M 650int a[M],n;int main(){ int i,j,cf; while(scanf("%d",&n)>0) {  if(n==-1) break;  printf("N=%d:n",n);  memset(a,0,sizeof(a));  a[0]=1;a[1]=2;  for(i=2;i<n;i++)  {   cf=0;   for(j=1;j<=a[0];j++)   {    a[j]*=i;    a[j]+=cf;    cf=a[j]/10;    a[j]%=10;   }   while(cf)   {    a[0]++;    a[a[0]]=cf%10;    cf/=10;   }  }  for(i=a[0];i;i--) printf("%d",a[i]);  printf("n"); } return 0;}