C :

#include <stdio.h>
#include <math.h>
int sum(int x,int y);
int zhi(int m);
 int main ()
{
 int x,y;
 while(scanf("%d %d",&x,&y)!=EOF)
 {
    printf("%d\n",sum(x,y));
 }
return 0;
}
int sum(int x,int y)
{
    int i;int count=0;
    for(i=x;i<=y;i++)
    {
        if(zhi(i)==1) count++;
    }
    return count;
}
int zhi(int m)
{
    int i;
    if(m==0||m==1) return 0;
    else if(m==2) return 1;
    else  if(m%2==0) return 0;
    else for(i=3;i<=sqrt(m);i+=2)
    {
        if(m%i==0) return 0;
         
    }
    return 1;
}

C++ :

#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <sstream>
#include <cstring>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <queue>
#include <stack>
#include <map>
#include <set>
#include <numeric>
#include <cassert>
#include <complex>
#include <ctime>

#define clr(x,a) memset(x,a,sizeof(x))
#define sz(x) (int)x.size()
#define rep(i,n) for(int i=0;i<n;i++)
#define repeat(i, a, b) for(int i=(a);i<=(b);i++)
#define all(v) (v).begin(), (v).end()
#define Unique(store) store.resize(unique(store.begin(),store.end())-store.begin())
#define X first
#define Y second



using namespace std;

const int N=1e5+10;

bool isp(int n) {
    for (int i=2;i*i<=n;i++) {
        if (n%i==0) return false;
    }
    return true;
}
bool v[N];
int main () {
    memset(v,0,sizeof v);
    for (int i=2;i<=100000;i++) v[i]=isp(i);
    int a,b;
    while (cin>>a>>b) {
        int ret=0;
        for (int i=a;i<=b;i++) ret+=v[i];
        cout<<ret<<endl;
    }
    return 0;
}