楼梯问题：一次最多跨两个阶梯，有多少种走法

采用排列组合的方法在C语言下实现。前24层结果和公布的是一致的，但25层以后出现分歧了，调试几次后没发现问题，测试环境下的int是32位的。疑惑，还请高人指点,显示结果如下：
NO20:10946
NO21:17711
NO22:28657
NO23:46368
NO24:73735
NO25:105676
NO26:162973
NO27:140125
NO28:254929
NO29:322365
NO30:449574
NO31:568275
NO32:-45160
NO33:249710
NO34:523678
NO35:969095
=======================源代码===============
#include<stdio.h>
#define N 50

int stage(int stages, int spansteps)
{
 int router = 0;
 int i = 0;
 for(i=0;i<=stages/spansteps;i++)
 {
  router += Combination(stages-i,i);
 }

 return router;
}

// C(Combination ) in Probability theory and mathematical statistics
int Combination (int n, int r)
{
 return (int)Arrangment(n,r)/Arrangment(r,r);
}

//A(Arrangment ) in Probability theory and mathematical statistics
int Arrangment(int n, int r)
{
 int product = 1;
 for(r;r>0;r--)
 {
  product *=n;
  n--;
 }

 return (int)product;
}

int Power(int base, int power)
{
 int result = 1;

 do
 {
  result *=base;
  power -- ;
 }
 while(1!=power);

 return result;
}

int main(void)
{ 
 int i = 20;
 while(i<=35)
 {
  printf("NO%d:%d\n",i,stage(i,2));
  i++;
 }
 printf("%d\n",Power(2,31));
 return 0;
}