> sum(2,7,5)
[1] 14

> x
[1]  2 NA  3  1  4

> sum(x)    # if any element is NA or NaN, result is NA or NaN
[1] NA

> sum(x, na.rm=TRUE)    # this way we can ignore NA and NaN values
[1] 10

> mean(x, na.rm=TRUE)
[1] 2.5

> prod(x, na.rm=TRUE)
[1] 24