class Solution {

public:

```
long long int C(int n , int k ) {
long long int res = 1;
for(int i=1;i<=k;i++){
res*=n;
res/=i;
n--;
}
return res;
}
long long int climbStairs(int n) {
int up = 0;
long long int ans = 0;
int twos = 0;
int all = 0;
while(up<n){
up+=2;
if(up>n) break;
twos = up/2;
all = (n - up) + twos;
ans+=C(all,twos);
}
return ans+1;
}
```

};