Longest distance between circumferences

#include <math.h>
#include <stdio.h>
#define N 5

typedef struct {
  double x;
  double y;
  double radius;
} Circumference;

double compute_distance(Circumference c1, Circumference c2);
double compute_longest_distance(Circumference circumferences[], size_t length);

int main(void) {
  Circumference circumferences[N];

  printf("Insert %d circumferences [x y radius]: ", N);
  for (size_t i = 0; i < N; i++) {
    scanf("%lf %lf %lf", &circumferences[i].x, &circumferences[i].y,
          &circumferences[i].radius);
  }

  printf("Longest distance: %.4lf", compute_longest_distance(circumferences, N));
  return 0;
}

double compute_distance(Circumference c1, Circumference c2) {
  // (x2 - x1) ^ 2 + (y2 - y1) ^ 2
  double sq_distance =
      (c1.x - c2.x) * (c1.x - c2.x) + (c2.y - c1.y) * (c2.y - c1.y);

  double abs_distance = sq_distance >= 0 ? sq_distance : -sq_distance;
  // sqrt( (x2 - x1) ^ 2 + (y2 - y1) ^ 2 )

  // Substract (c1.radius + c2.radius) for distance between borders
  return sqrt(abs_distance);
}

double compute_longest_distance(Circumference circumferences[], size_t length) {
  if (length <= 0) return -1.0;
  if (length == 1) return 0.0;

  double longest_distance = 0.0;

  for (size_t i = 0; i < length; i++) {
    for (size_t j = i + 1; j < length; j++) {

      // Computing distance between circumferences[i] and circumferences[j]
      double current_distance =
          compute_distance(circumferences[i], circumferences[j]);

      // Longer then the previous best
      if (current_distance > longest_distance) {
        longest_distance = current_distance;
      }
    }
  }

  return longest_distance;
}