#include <bits/stdc++.h>
using namespace std;
int main()
{
    double x, n, p;
    cin >> x >> n;
    for (int i = 1; i <= n; i++)
    {
        p = x * 0.001;
        x += p;
    }
    cout << fixed << setprecision(4) << x;
    return 0;
}

不用循环

#include<bits/stdc++.h>
using namespace std;
int main()
{
    double x,n;
    cin>>x>>n;
    x*=pow(1.001,n);
    cout<<fixed<<setprecision(4)<<x;
    return 0;
}

#include<bits/stdc++.h>
using namespace std;
int main()
{
    double x,n;
    cin>>x>>n;
    while (n--)
        x*=1.001;
    cout<<fixed<<setprecision(4)<<x;
    return 0;
}

#include <bits/stdc++.h>
using namespace std;
int main()
{
    int n;
    double x;
    cin >> x >> n;
    for (int i = 1; i <= n; i++)
    {
        x += x * 0.001;
    }
    cout << fixed << setprecision(4) << x;
    return 0;
}