求解：为什么把product=0改为product=1程序就可以正常运行。求原理。

#include
double fact (int n);
int main (void)
{
int m, n;
double p;

printf("Enter m,n:\n");
scanf ("%d%d", &m, &n);
  p = fact(n)/fact(n-m);
printf ("result = %.0f\n", p);

return 0;

}

double fact (int n)

int i;
double product;

product = 0;
for (i = 1; i <= n; i++){
    product = product * i;
}

return product;

}

product = 0;，后面所有乘法结果为0了