limit.c

#include <stdio.h>
#include <math.h>

#define ITERATIONS 10000000

double eval(double x, double res) {
  unsigned i;
  for (i = ITERATIONS; i > 1; i--) {
    res = pow(x - res, 1.0 / i);
  }
  return res;
}

int main(int argc, char **argv) {
  double x, res;
  sscanf(argv[1], "%lf", &res);
  sscanf(argv[2], "%lf", &x);
  res = eval(x, res);
  printf("%.14lf\n", res);
}