Addition of Diagonal Elements of a matrix using C

#include<stdio.h>
main()
{
int i,j,a[10][10],sum,m,n;

printf("\nEnter the number of Rows : ");
scanf ("%d",&m);

printf("\nEnter the number of Columns : ");
scanf ("%d",&n);

for(i=0;i<m;i++ )
       for(j=0;j<n;j++)
       {
       printf("\nEnter the Element a[%d][%d] : ", i , j);
       scanf("%d",&a[i][j]);
       }

sum = 0;

for(i=0;i<m;i++ )
       for(j=0;j<n;j++)
       {
        if ( i == j )
        sum = sum + a[i][j];
       }
printf("\n Result after adding Diagonal Elements: %d\n",sum);
}