PROGRAMMING:This problem requires programming to calculate the sum of the first n items of the sequence 1 + 1 + 2 + 3 + 5 +

This problem requires programming to calculate the sum of the first n (n > = 1) items of the sequence 1 + 1 + 2 + 3 + 5 +.
###Input format:
The input gives a positive integer n on one line.
###Output format:
Output in the format of "sum = s" in one line
###Input example:
Here is a set of inputs. For example:
```in
three
```
###Output example:
The corresponding output is given here. For example:
```out
sum = 4
```







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