Consider the following C program carefully. What will be printed on the screen when this program is compiled and executed?

#include <stdio.h>
int main() {
    int a[] = {3, 6, 9, 13};
    int i;
    for (i = 1; i < 4; i++)
        a[i] = a[i] + a[i - 1];
    printf("%d", a[3]);
    return 0;
}