[luogu_p4820] [national training team] stack [mathematics] [physics] [harmonic progression]

Ideas ：

First, through physical knowledge , We know it's a request
$\frac{m}{2}\ast (1+\frac{1}{2}+\frac{1}{4}+\frac{1}{6}\dots \dots +\frac{1}{n})$
This thing inside is destroyed by harmonic series .
Note that the final answer is not available , So take one down

$code$

#include<iostream>
#include<cstdio>
#include<cmath>

using namespace std;

const double eps = 1e-6;
const double euler = 0.5772156649015328;

long long n, m;

int main() {
    
	scanf("%lld%lld", &n, &m);
	if(n<=1e7) {
    
		double s=0;
		for(int i = 1; i <= n; i ++)
			s=s+(1.0/i);
		cout<<round(s * m / 2 - 0.5 - eps);
	}
	else {
    
		double s = (log(n) + euler) * m / 2 - 0.5 - eps;
		cout<<round(s);
	}
	return 0;
}