题目:


解题思路:


(图源CSL的队友)

ac代码:


#include <bits/stdc++.h>
using namespace std;
const int mod = 1000000007;
long long quickpow(long long a, long long b) {
    if (b < 0) return 0;
    long long ret = 1;
    a %= mod;
    while(b) {
        if (b & 1) ret = (ret * a) % mod;
        b >>= 1;
        a = (a * a) % mod;
    }
    return ret;
}
long long inv(long long a) {
    return quickpow(a, mod - 2);
}
int main() {
    //freopen("/Users/zhangkanqi/Desktop/11.txt","r",stdin);
    long long  n;
    scanf("%lld",&n);
    long long ans=((n+1)*inv(2*n))%mod;
    printf("%lld",ans);
    return 0;
}