How to use if statement in c programming

In the program below, we will learn about how the if statement works. I wanted to calculate square root value of a given number if and only if it is greater than equal to zero. If, however, the value of given number lowers than zero, it always print a zero.    

       /*To verify the if statement */
        #include<stdio.h>
        #include<math.h>
        int main()
        {
            float a, b;
            printf("Enter the value of a:");
            scanf("%f", &a);
            if (a>=0)
            b=sqrt(a);
            printf("\n The square root of %f is: %f\n", a,b);
            return(0);
        }