思路：定义一个函数 连续用三次就可以啦
#include <stdio.h>
double f (int n) {
double s = 1;
for(int i = 1;i < n + 1;i ++){
s *= i;
}
return s;
}
int main(){
int m,n;
scanf("%d%d",&m,&n);
int c = n - m;
printf("result = %.0f",f(n)/(f(m)*f(c)));
}