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

int specialNo(int a[], int n)
{
    int x = 0;

    for (int i = 0; i < n; i++)
    {
        // xoring all elements outputs the special number
        x ^= a[i];
    }

    // return the special number
    return x;
}

int main()
{
    int x, count = 0;

    // array of positive no initialized to -1 for now
    int a[20] = {-1};

    printf("enter positive numbers  ending with -1\n");
    scanf("%d", &x);

    // first no
    a[0] = x;
    while (x != -1)
    {
        scanf("%d", &x);
        // from second no onwards
        a[count + 1] = x;
        count++;
    }

    // function to check duplicate exists or not
    printf("\nthe special number occuring odd no of times is %d\n", specialNo(a, count));

    return 0;
}

Output

enter positive numbers  ending with -1
5
4
3
5
9
4
7
-1

the special number occuring odd no of times is 13