Detrimental of center trig and sub trig of any matrix

gistfile1.c

#include<stdio.h>

//Created By Ahmed Bouhuolia
main(){
	int i, j, n, m, matrix[256][256], sum;
	
	printf("Enter the number of rows and columns of the matrix ");
	scanf("%d %d", &n, &m);
	
	printf("Enter the elements of the matrix %d*%d ", n, m);
	
	for( i = 0; i < n; i++ ){
		for( j = 0; j < m; j++){
			scanf("%d", &matrix[i][j]);
		}
	}
	
	for( i = 0; i < n; i++ ){
		for( j = 0; j < m; j++ ) {
			if( i + j == m - 1 ){
				sum = sum + matrix[i][j];
			}
			if( i == j){
				sum = sum + matrix[i][j];
			}
		}
	}
	
	printf("%d", sum);
}