getting wrong answer in SPOJ problem UCV2013J

here is my code

/*
Valences
submitted by 
Arjun Mishra
*/
#include<iostream>
#include<string.h>
#include<math.h>
using namespace std;
long long int A;
long long int compare[21];
void function()
{long long int i;
compare[0]=1;
for(i=1;i<=20;i++)
compare[i]=(long long int)pow(2,i);
     }
int main()
{long long int N,i,a,sum=0;
function();
//for(i=0;i<=20;i++)
//cout<<compare[i]<<endl;
while(1)
{sum=0;
 cin>>N;
 if(N==0)
 return 0;
 for(i=0;i<=20;i++)
 {
 if(N>=compare[i]&&N<compare[i+1])
 {a=compare[i];
 //cout<<i<<endl;
 break;}              
 }
 //cout<<a<<endl;
 for(i=1;i<=a-1;i++)
 cin>>A;
 for(i=a;i<=N;i++)
 {cin>>A;
 sum+=A;
 //cout<<sum<<endl;
 }
 cout<<sum<<endl;
 }
 }

i don't know why it is producing WA after 50s of execution