c – 当我尝试为我的程序提供3个输入时,它需要4才能正常运行.这是为什么？

#include <stdio.h>
#include <stdlib.h> 
#define f(X) (1 / (x*x+1))



int main(){
    double a,b,h,x,y;

    printf("Enter a,h:  ");
    scanf(" %lf %lf %lf ",&a,&b,&h);

// I ask for 3 inputs but the progrAMM needs 4 to run...why is that?


    x = a;

     while(x<b)
     {

        y = f(X);
        printf("%lf %lf \n",y );
        x +=h;

     }


    system("Pause");
    return(0);  

}