C :

#include <stdio.h>

int fact(int n);

int main(void) {
    int m,n,sum;
    scanf("%d %d",&m,&n);
    printf("%d\n",(fact(m))/(fact(n))/(fact(m-n)));
	return 0;
}

int fact(int n){
	int i=1;
	int sum=1;
	while(i<=n){
		sum*=i;
		++i;
	}
	return sum;
}

C++ :

#include<iostream>
#include<cstdio>
using namespace std;
float fact(int n)	//求阶乘
{
	if (n==0 || n==1) return 1;
	float t=1;
	for (int i=1; i<=n; i++) t*=i;
	return t;
}
int main()
{
	int m,n;
	cin>>m>>n;
	printf("%.0f\n",fact(m)/(fact(n)*fact(m-n)));
	return 0;
}