1615 . A! + B! problem

#include <bits/stdc++.h>

using namespace std;

#define int long long

#define maxn 1000000

bitset < maxn > lib;

vector < int > prime;

signed main(){

    ios::sync_with_stdio ( false );

    lib[0] = lib[1] = true;

    for ( int i = 2 ; i < maxn ; i++ ){

        if ( !lib[i] ){

            prime.push_back ( i );

            for ( int j = i ; j < maxn ; j += i )

                lib[j] = true;

        }

    }

    int a, b, len = prime.size();

    int ans, tmp;

    while ( cin >> a >> b ){

        if ( a > b ) swap ( a, b );

        //prime that less than A

        ans = upper_bound ( prime.begin(), prime.end(), a ) - prime.begin();

        tmp = 1;

        for ( int i = a + 1 ; i <= b ; i++ )  tmp *= i;

        ++tmp;

        for ( int i = 0 ; i < ans ; i++ )

            while ( tmp % prime[i] == 0 ) tmp /= prime[i];

        //B! / A! +1 -> prime

        for ( int i = ans ; i < len ; i++ ){

            if ( tmp % prime[i] == 0 ){

                ++ans;

                while ( tmp % prime[i] == 0 ) tmp /= prime[i];

            }

        }

        // if prime too large

        if ( tmp > maxn ) ans++;

        cout << ans << '\n';

    }

}