Need help modifying my C program to calculate error in rk4 method!!

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

#define N 2                                   
#define MIN 0.0
#define K 1            /* spring constant */
#define M 1            /* mass of block */


void runge4(double x, double y[], double step);
double f(double x, double y[], int i);

main()

{   
   double x, y[N], v0, x0, tmax, dt;
   int j, n;
   
   FILE *output;                             /* save data in pa2.dat */
   output = fopen("pa2.dat","w");

   printf("\nPlease Enter initial position and velocity X0 and V0: ");
   scanf("%le %le", &x0, &v0);

   printf("\nPlease Enter Tmax and N: ");
   scanf("%le %d", &tmax, &n);

   dt = tmax/(n-1);
   
   y[0] = x0;                                /* initial position  */
   y[1] = v0;                                /* initial velocity  */

   
   fprintf(output, "%f\t%f\n", x, y[0]);

   for(x = MIN; x <= tmax ; x += dt)
   {
      runge4(x, y, dt);
      fprintf(output, "%f\t%f\n", x, y[0]);   /* position vs. time */
   }
   printf("data stored in rk4.dat\n\n");
   fclose(output);
}



void runge4(double x, double y[], double step)

{

   double h=step/2.0,                        
          t1[N], t2[N], t3[N],               
          k1[N], k2[N], k3[N],k4[N];
         
   int i;
 
   for (i=0; i<N; i++) t1[i] = y[i]+0.5*(k1[i]=step*f(x, y, i));
   for (i=0; i<N; i++) t2[i] = y[i]+0.5*(k2[i]=step*f(x+h, t1, i));
   for (i=0; i<N; i++) t3[i] = y[i]+    (k3[i]=step*f(x+h, t2, i));
   for (i=0; i<N; i++) k4[i] =                 step*f(x + step, t3, i);
      
}


/* equations defined */

double  f(double x, double y[], int i)
{
   if (i == 0) return((2*y[0]*(1 - y[1]));               /* RHS of first equation */
   if (i == 1) return((-y[1]*(1 - y[0]));        /* RHS of second equation */
}