#include <stdio.h>
#include <math.h>
int main()
{
	long long  x = 0;
	long long z = 0;
	long  double c = 0;
	scanf("%d %d ", &x, &z);
	scanf("%llf",&c);
	long  double b = x - 2 * z;
	printf("%.lf\n", ceil(b/c));
	return 0;
}