A very slow program

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

#define RUNS 1000
#define SIZE 1000000

int mark[SIZE];

int main(void)
{
     time_t start, finish;
     int i, loop, n, num;

     time(&start);

     /*  This loop finds the prime numbers between 2 and SIZE   */
     for (loop = 0; loop < RUNS; ++loop)
     {
         for (n = 0; n < SIZE; ++n)
             mark[n] = 0;
         /*  This loops marks all the composite numbers with -1  */
         for (num = 0, n = 2; n < SIZE; ++n)
             if (!mark[n])
             {
                 for (i = 2 * n; i < SIZE; i += n)
                     mark[i] = -1;
                 ++num;
             }
     }
     time(&finish);
     printf("Program takes an average of %f seconds "
            "to find %d primes.\n",
            difftime(finish, start) / RUNS, num);
}
/*
The result on my slow machine:
Program takes an average of 0.018000 seconds to find 78498 primes.
*/