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THX..
#include <stdlib.h>
#include <stdio.h>
#include <math.h>
long double f1[2001],f2[2001];
int main()
{
    int x , y , fx , fy , n ;
    double fn;
    scanf("%d%d%d%d%d",&x,&fx,&y,&fy,&n);
    f1[0] = 1 ; f1[1] = 0 ;
    f2[0] = 0 ; f2[1] = 1 ;
    if ( x > y )
    {
     int  tmp = x ;
       x = y ;
       y = tmp ;
       tmp = fx;
       fx = fy;
       fy = tmp;
    }
    int k ;
    int i;
    for ( i=x+2; i <= y ; i ++ )
    {
      if ( x < 0 ) k = -x + i;
      else k = i - x;
      f1[k] = f1[k-1]+f1[k-2];
      f2[k] = f2[k-1]+f2[k-2];
    }

    long double b = ceil(fy-f1[y-x]*fx)/(f2[y-x]);
    long double a = fx;
    long double c;
 if ( x+2 <= n )
 {
    for ( i = x+2 ; i <= n ; i ++ )
    {
        c = a + b ;
        a = b ; b = c ;
    }
    printf("%.0llf\n",c);
 }
 else
 {
   if (x == n ) printf("%.0llf\n",a);
    else if ( n == x + 1 ) printf("%.0llf\n",b);
       else
       {
          for (i = x ; i > n ; i -- )
           {
              c = b - a ;
              b = a ; a = c;
           }
          printf("%.0llf\n",c);
       }

 }

    system("PAUSE");
    return 0;
}

I did similarly in C++ and was getting WA on #10. Later, I wrote the same solution in Java (with BigInteger) and worked.