Find sum of diagonal elements of the given matrix

Program:

#include<stdio.h>
#include<conio.h>
void main()
{
    int mat[10][10];
    int i, j, size, sum = 0;
    clrscr();
    printf("Enter size of the square matrix : ");
    scanf("%d", &size);
    printf("\nEnter the elements of the matrix : \n\n");
    for(i = 0; i < size; i++)
	{
    	for(j = 0; j < size; j++)
		{
    		scanf("%d", &mat[i][j]);
		}
	}
    for(i = 0; i < size; i++)
	{
    	sum = sum + mat[i][i];
    }
	printf("\n The sum of diagonal elements in the matrix is : %d", sum);
    getch();
}

Output:

Enter size of the square matrix : 3
Enter the elements of the matrix :
1 2 3
4 5 6
7 8 9
The sum of diagonal elements in the matrix is : 15