11440 - Help Tomisu(计数问题)

细节参见代码:

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int mod = 100000007;
const int maxn = 10000000+5;
int T,kase=0,vis[maxn] = {0};
ll n,m,k,phi[maxn];
void sieve(int nn) {
    int m = sqrt(nn+0.5);
    for(int i=2;i<=m;i++) if(!vis[i])
        for(int j=i*i;j<=nn;j+=i) vis[j] = 1;
}
int main() {
    sieve(10000000);
    phi[1] = phi[2] = 1;
    for(int i=3;i<=10000000;i++)
        phi[i] = phi[i-1] * (vis[i] ? i : i-1) % mod;
    while(~scanf("%lld%lld",&n,&m)&&n) {
        ll ans = phi[m];
        for(int i=m+1;i<=n;i++) ans = ans * i % mod;
        printf("%d\n",(ans-1+mod)%mod);
    }
    return 0;
}

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