差点忘了while的用法：

#include<iostream>
using namespace std;

int main(){
    double n;
    cin>>n;
    int ji=1;
    double m=2.00,k=2.00;
    while(m<n){
        k*=0.98;
        m+=k;
        ji++;   
    }
    cout<<ji;
}

#include<iostream>
using namespace std;

int main(){
    double n;
    cin>>n;
    int sum=1;
    double m=2.00,k=2.00;
    while(m<n){
        k*=0.98;
        m+=k;
        sum++;   
    }
    cout<<sum;
}