Python的哦~

x=int(input())
y=x
x1=y
a=0
while x*0.001<x1:
    x1=x1*0.7
    if x*0.001<x1:
        y=y+2*x1
y=round(y,2)
print(y)

#include<bits/stdc++.h>
using namespace std;
int main()
{
	double n;
	cin >> n;
	double b = n;
	double S = n;
	for (int i=1; i <= 99999; i++)
	{
		if (pow(0.7,i)*n <= b / 1000)
			break;
		else
		{
			S = S + 2 * pow(0.7, i) * n;
			
		}
	}
	printf("%.2f", S);
}

C++ :

#include <iostream>
#include <cstdio>
#include <cstring>
#include <stdlib.h>
using namespace std;

int main()
{
    double a,sum=0,b;
    scanf("%lf",&a);
    b=a;
    for(int i=0;;i++){

            sum+=b;
            b*=0.7;
    if(b<(a/1000))break;
            sum+=b;

    }
    printf("%.2lf",sum);

    return 0;
}