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| #include<iostream>
#include<cstdio>
#include<cmath>
#include<cstring>
#include<algorithm>
using namespace std;
#define int ll
#define REP(i,e,s) for(register int i=e; i<=s; i++)
#define DREP(i,e,s) for(register int i=e; i>=s; i--)
#define ll long long
#define DE(...) fprintf(stderr,__VA_ARGS__)
#define DEBUG(a) DE("DEBUG: %d\n",a)
#define file(a) freopen(a".in","r",stdin);freopen(a".out","w",stdout)
int read() {
int x=0,f=1,ch=getchar();
while(ch>'9'||ch<'0'){if(ch=='-')f=-1;ch=getchar();}
while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}
return x*f;
}
const int MOD=1e9+7,MAXN=1e7+10;
int vis[MAXN],num,fac[MAXN],inv[MAXN];
int qpow(int a,int b) {
int ans=1;
while(b) {
if(b&1) ans=ans*a%MOD;
a=a*a%MOD;
b>>=1;
}
return ans;
}
int c(int n,int m) {
if(n<m) return 0;
return fac[n]*inv[m]%MOD*inv[n-m]%MOD;
}
signed main() {
int l=read(),r=read(),n=r-l+1;
REP(i,l,r) {
if(vis[i]) continue;
num++;
for(int j=i+i; j<=r; j+=i) vis[j]=1;
}
fac[0]=fac[1]=1;
REP(i,2,n) fac[i]=fac[i-1]*i%MOD;
inv[n]=qpow(fac[n],MOD-2);
DREP(i,n-1,0) inv[i]=inv[i+1]*(i+1)%MOD;
int ans=0;
REP(i,1,n) ans=(ans+i*fac[i-1]%MOD*num%MOD*c(n-num,n-i)%MOD*fac[n-i]%MOD)%MOD;
printf("%lld\n",ans);
return 0;
}
|
