输入一个数字求阶乘

#include<stdio.h>
main()
{
int a,n;
float m=1;
printf("input a number\n");
scanf("%d",&a);
if(a==1||a==0)
printf("output is 1\n");
else
{
for(n=2;n<=a;n++)
{  
m=m*n;
}
printf("output is %.0f\n",m);
}

}

input a number
1
output is 1

input a number
 20
output is 2432902023163674600