if (x.position == y.position) {

return x.position < y.price;

} else {

return x.position < y.position;

}

int capacity, distance, average, number;

while (scanf("%d%d%d%d", &capacity, &distance, &average, &number) != EOF) {

double maxDistance = capacity * average;

for (int i = 0; i < number; ++i) {

scanf("%lf%lf", &arr[i].price, &arr[i].position);

}

arr[number].price = 0;

arr[number].position = distance;

sort(arr, arr + number, Compare);

if (arr[0].position != 0) {

printf("The maximum travel distance = %.2f\n", 0.0);

continue;

}

double position = arr[0].position;

double price = arr[0].price;

int index = 0;

double gas = 0;

double answer = 0;

bool reach = true;

while (position != distance) {

bool flag = false;

double newPrice = INF;

int newPosition = 0;

for (int i = index + 1; i <= number && arr[i].position <= position + maxDistance; ++i) {

if (arr[i].price <= price) {

answer += price * ((arr[i].position - position) / average - gas);

gas = 0;

index = i;

price = arr[i].price;

position = arr[i].position;

flag = true;

break;

} else if (arr[i].price <= newPrice) {

newPrice = arr[i].price;

newPosition = arr[i].position;

}

}

if (!flag && newPrice != INF) {

answer += price * (capacity - gas);

gas = capacity - (newPosition - position) / average;

price = newPrice;

position = newPosition;

}

if (!flag && newPrice == INF) {

printf("The maximum travel distance = %.2f\n", position + maxDistance);

reach = false;

break;

}

}

if (reach) {

printf("%.2f\n", answer);

}

}

return 0;