P2209 [USACO13OPEN] Fuel Economy S

Farmer John has decided to take a cross-country vacation. Not wanting his cows to feel left out, however, he has decided to rent a large truck and to bring the cows with him as well! The truck has a large tank that can hold up to G units of fuel (1

第一行： 四个整数：$N, G, B, D$。 接下来 $N$ 行：每一行都有两个整数 $X_i$ 与 $Y_i$，意义如上所述。

一个整数，如果 FJ 无法到达旅途的终点，那么输出 `-1`，否则输出 FJ 到达目的地时花费的油费用的最小值。

样例解释：FJ 先移动 $2$ 个单位，然后停下购买 $2$ 个单位的油（要花费 $40 \times 2$）。然后一直前进到距离起点 $5$ 个单位的地方，此时油箱为空。这时向油箱里加满油（要花费 $7 \times 10$）。再向前走 $5$ 个单位，加 $2$ 个单位的油（花费 $12 \times 2$）。最后一直走到终点。此时总花费是 $174$。