Assuming you don't want to add them and store the result in a new variable. To print them concatenated to each other like 15, you may write something like this.

printf("%d%d\n", x, y);

To add them and store the result in a new variable, you may multiply x by 10 first, then add them and store the result in a new int variable

int sum = (x * 10) + y;
printf("%d\n", sum);

Assuming you don't want to add them and store the result in a new variable. To print them concatenated to each other like 15, you may write something like this.

printf("%d%d\n", x, y);

To add them and store the result in a new variable, you may multiply x by 10 first, then add them and store the result in a new int variable

int sum = (x * 10) + y;
printf("%d\n", sum);

Update: in case you have an array of 4 ints and you want to add them in such a way as the first, the second, the third, the fourth elements are positioned in the ones, tens, hundreds, thousands places respectively, you may write something like this

int arr[] = {2, 0, 1, 4};
int position = 1;
int year = 0;

for (int i = 3; i >= 0; i--)
{
    year += arr[i] * position;
    position *= 10;
}

After the loop ends execution, year should be 2014.

I assume you don't wanna add them and store them in a new variable. To print them out as 15, you may write something like this.

printf("%d%d\n", x, y);

To add them and store the result in a new variable, you may multiply x by 10 first, then add them

int sum = (x * 10) + y;
printf("%d\n", sum);

Assuming you don't want to add them and store the result in a new variable. To print them concatenated to each other like 15, you may write something like this.

printf("%d%d\n", x, y);

To add them and store the result in a new variable, you may multiply x by 10 first, then add them and store the result in a new int variable

int sum = (x * 10) + y;
printf("%d\n", sum);