#include <stdio.h>
float func(float x,float y)
{ return(x-y);
}
float runge_kutta(float x0,float xn,floaty0,int N)
{ float x,y,y1,y2,h,xh;
float d1,d2,d3,d4;
int i;
x=x0;
y=y0;
h=(xn-x0)/(float)N;
for(i=1;i<=N;i++)
{ xh=x+h/2;
d1=func(x,y);
d2=func(xh,y+h*d1/2.0);
d3=func(xh,y+h*d2/2.0);
d4=func(xh,y+h*d3);
y=y+h*(d1+2*d2+2*d3+d4)/6.0;
x=x0+i*h; }
return(y);
}