I don't think that problem is with double,mine solution pass with using double.May be you have not used appropriate precision.

Use double for floating-point computations; float usually does not have enough precision, and the cost of double-precision calculations versus single-precision is negligible. In fact, on some machines, double-precision operations are faster than single. The precision offered by long double usually is unnecessary and often entails considerable run-time cost.

From C++ Primer, 5e

From my experience, you should never use long double. It's slower and completely useless since double precision is always enough if you are careful enough. You probably have some precision issue here.

UPD: Oh, I see now why you got WA.
Since the sum of all calculation don't have any precision or floating point value, precision is coming only when dividing by n
That statement is very far from true. Precision in floating type comes not from numbers after floating point — precision exists because number of meaningful digits that can be stored in double is limited. You can store numbers up to 10³⁰⁸ because double also stores power of 10 separately. double tot stores numbers up to 10¹⁸ in your problem, and it simply can't store all of ~64 digits that can exists, so there goes significan't precision issue. Yeah, on of the way to dodje this is to use long double.

Rule 1 of floating point numbers — don't use floating point numbers.

Only place in this task that requires floating point calculations is division by n at the end.

Here is a AC solution using float http://codeforces.net/contest/1009/submission/40386856.