C :

#include <stdio.h>

int is_prime( int t ) {
    if( t == 0 || t == 1 ) return 0 ; //1 是特殊情况

    for( int i = 2 ; i * i <= t ; i ++ ) {
        if( t % i == 0 ) return 0 ;
    }
    return 1 ;
}

int main() {

    int a, b ;
    while( scanf("%d%d", &a, &b) != EOF ) {
        if( a == 0 && b == 0 ) break ;
        if( a > b ) {
            int t = a ;
            a = b ;
            b = t ;
        }

        int count = 0 ;
        for( int i = a ; i <= b ; i ++ ) {
            if( is_prime(i) == 1 ) {
                count ++ ;
            }
        }
        printf("%d\n", count) ;

    }

    return 0 ;
}

C++ :

#include<iostream>
#include<cstdio>
using namespace std;
int is_prime(int n)
{
    int i ;
    if(n < 2) return 0;
    if(n == 2)return 1;
    for(i = 2; i <= n/2; i++)
    if(n % i == 0) return 0;
    return 1;
}
int main()
{
    //freopen("5.txt","r",stdin);
    //freopen("6.txt","w",stdout);
    int n,m,i,j;
    while(cin>>n >>m)
    {
        if(n == 0 && m == 0)break;
        if(n > m)
        {
            i = n;
            n = m;
            m = i;
        }
        j = 0;
        for(i = n; i <= m; i++)
        {
            if(is_prime(i))
            j++;
        }
        cout<< j<<endl;
    }
    return 0;
}