求修改

谁知道我的代码哪里错了

#include <iostream>
#include <cmath>
using namespace std;
double log3(double x){
    return log(x)/log(3.0);
}
int main(){
    int a;
    cin >>a;
    double n=log3(2*a+1);
    int r=ceil(n);
    cout <<r;
    return 0;
}