accumulate in cpp

accumulate(first, last, sum);
first, last : first and last elements of range 
              whose elements are to be added
sum :  initial value of the sum
    accumulate(start, end, initial_sum);
accumulate(first, last, sum, myfun); 
myfun : a function for performing any 
        specific task. For example, we can
        find product of elements between
        first and last.