# Not able to proceed with Series sum Code Answer

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I wanted to find the missing number in series so i thought a simple idea why not add all the numbers in array which are in series and hold it in one variable and then calculate the sum of series by formula `Sn=n/2(a+l)` but while calculating the series sum i am getting some error.

```public class Missing {
public static void main(String[] args) {

int ar [] = {1,2,3,4,5,6,7,8,9,10,11,12,13};
int sum = 0; int total=0;
for(int num: ar)
{
sum = sum+num;
}

int n = ar.length;
int a = ar[0];
int l =ar[ar.length-1];
total = [n/2*(a+l)];

System.out.print("The missing number is "+(sum-total));

}}
```

total = [n/2*(a+l)]; ……………………….(1)

This is where i am getting error.

enter image description here

The first thing is in `total = [n/2*(a+l)];` `[]` is not valid syntax in this context. The second thing I noticed, is that your formula to calculate the sum seems odd, maybe you meant `Sn = (n * (a + l)) / 2?`. After making those two changes the code should look as follows:

```public class Missing {
public static void main(String[] args) {

int ar [] = {1,2,3,4,5,6,7,8,9,10,11,12,13};
int sum = 0;
for(int num: ar)
{
sum = sum+num;
}

int n = ar.length;
int a = ar[0];
int l =ar[ar.length - 1];
int total = (n * (a + l)) / 2;
System.out.print("The missing number is "+(sum - total));
// outputs 0 which is correct nothing is missing

// Now if you remove say 12 from the array
// by changing the array to int ar [] = {1,2,3,4,5,6,7,8,9,10,11,0,13};
// you should get back -12 which means 12 is missing
}
}
```