Can some one please help as to why my solution gives a segmentation fault?

include

include

include

include

int main() {

unsigned long long int n,m,l,b,k,i,val=0;
scanf("%llu%llu",&n,&m);
unsigned long long int a[n+1];
for(i=1;i<=n;i++)
{
   a[i]=0;    
}
while(m--)
{
    scanf("%llu%llu%llu",&l,&b,&k);        
    for(i=l;i<=b;i++)
    {
       a[i]+=k;    
       if(a[i]>val)
       {
           val=a[i];
       }
    }
}
printf("%llu",val);
return 0;

}