#include <stdlib.h>
    #include <stdio.h>
    #include <time.h>

    int clock_gettime(clockid_t clk_id, struct timespec *tp);
    int main(int argc, char *argv[]) {

    struct timespec {
      time_t   tv_sec;  /* seconds */
      long     tv_nsec;   /* nanoseconds */
    };

    struct timespec tps, tpe;
    clock_gettime(CLOCK_REALTIME, &tps);

          float xp = 2;
          float xk = 5;
          float calka = 0;
          float dx = 0;
          int i = 0;
          int n = 3;
          dx = (xk - xp) / (float)n;

          calka = 0;
          for (i = 1; i < n; i++) {
            calka += (xp + i * dx)*(xp + i * dx);
          }
          calka += ((xp*xp) + (xk*xk)) / 2;
          calka *= dx;
    clock_gettime(CLOCK_REALTIME, &tpe);
    printf("%lu s, %lu ns\n", tpe.tv_sec-tps.tv_sec,
        tpe.tv_nsec-tps.tv_nsec);
          return 0;
    }