Adjacency matrix

Program for creation of adjacency matrix
#include<stdio.h>
#define max 20
int adj[max][max];
/*Adjacency matrix */
int n;
    /* Denotes number of nodes in the graph */
main()
{
int max_edges,i,j,origin,destin;
char graph_type;
printf("Enter number of nodes : ");
scanf("%d",&n);
printf("Enter type of graph, directed or undirected (d/u) : ");
fflush(stdin);
scanf("%c",&graph_type);
if(graph_type=='u')
max_edges=n*(n-1)/2;
else
max_edges=n*(n-1);
for(i=1;i<=max_edges;i++)
{
printf("Enter edge %d( 0 0 to quit ) : ",i);
scanf("%d %d",&origin,&destin);
if( (origin==0) && (destin==0) )
break;
if( origin > n || destin > n || origin<=0 || destin<=0)
{
printf("Invalid edge!\n");
i--;
}
else
{
adj[origin][destin]=1;
if( graph_type=='u')
adj[destin][origin]=1;
}
}/*End of for*/
printf("The adjacency matrix is :\n");
for(i=1;i<=n;i++)
{
for(j=1;j<=n;j++)
printf("%4d",adj[i][j]);
printf("\n");
}
}/*End of main()
*/