return a value from the function

#include<stdio.h>
long long a[1000000],i,ans,n;
long long mon(long long n)
{
   if(n<1000000)
       return a[n];
   else
       return mon(n/2)+mon(n/3)+mon(n/4);
}
int main()
 {
   a[0]=0;a[1]=1;a[2]=2;a[3]=3;a[4]=4;a[5]=5;
   for(i=6;i<1000000;i++)
   {
      a[i]=a[i/2]+a[i/3]+a[i/4];
      if(a[i]<i)
          a[i]=i;ON
    }
while(scanf("%lld",&n)!=EOF)
    {
    ans=mon(n);
    printf("%lld\n",ans);
    }
return 0;
}

CAN ANYONE EXPLAIN ME THIS CODE FOR THE PROBLEM "COINS" .... AND PLZ EXPLAIN THE RETURN VALUE OF THE FUNCTION MON