Question about WM_MOUSEWHEEL

#include <stdio.h>
#include<string.h>

char f[10000];
char factorial[1010][10000];

void multiply(int k)
{
int cin,sum,i;
int len = strlen(f);
cin=0;
i=0;
while(i<len)
{
sum=cin+(f[i] - '0') * k;
f[i] = (sum % 10) + '0';
i++;
cin = sum/10;
}
while(cin>0)
{
f[i++] = (cin%10) + '0';
cin/=10;
}
f[i]='\0';
for(int j=0;j<i;j++)
factorial[k][j]=f[j];

factorial[k][i]='\0';
}
void fac()
{
int k;
strcpy(f,"1");
for(k=2;k<=1000;k++)
multiply(k);
}
void print(int n)
{
int i;
int len = strlen(factorial[n]);
printf("%d!\n",n);
for(i=len-1;i>=0;i--)
printf("%c",factorial[n][i]);
printf("\n");
}
int main()
{
int n;
factorial[0][0]='1';
factorial[1][0]='1';
fac();
while(scanf("%d",&n)==1){
print(n);
}
return 0;
}