How to find exact execution time of java math library functions in nanosecond range?

 I have used System.nanoTime() to find the time but it's giving different execution time on each run.

 code that i have used is:

import java.util.concurrent.TimeUnit;
import java.lang.Math; 

class TimeUtil
{
public static void main(String[] args) throws InterruptedException {

    long startTime = System.nanoTime();

    /* ... the code being measured starts ... */

    Math.sqrt(5.625);

    /* ... the code being measured ends ... */

    long endTime = System.nanoTime();

    // get difference of two nanoTime values
    long timeElapsed = endTime - startTime;

    System.out.println("Execution time in nanoseconds  : " + timeElapsed);

    System.out.println("Execution time in milliseconds : " + 
                            timeElapsed / 1000000);
}
}