题目链接：

网上的解答铺天盖地。我硬是花了两天才懂了点。

wiki上的解释：

 

解答：每个点的油量是500,500*2,500*3... ...

每次的距离是500，500/3，500/5，... ...，500/2n-1;

#include 
     
      using namespace std;int main(){    double dist, tank;    scanf("%lf%lf", &dist, &tank);    double now = dist, cost = 0, pre;    int k = 1;    while( (pre = now - tank / (2*k-1)) > 0){        cost += tank;        now = pre;        k++;    }    cost += now * (2*k-1);    printf("%.0f\n", ceil(cost));    return 0;}