get the sum of all prime numbers from 1 to N

int sum=0;
int a[N]={0};

for(int i=2;i < N;i++)
{
if(a[i]==0)
sum+=i;
for(int j=i;j < N/i;j++)
{
if(a[j]==0||a[j]==i)
a[i*j]=i;
}
}

return sum;