uva 10763 Foreign Exchange

#include<stdio.h>
#include<string.h>

int main()
{
    int map[1024][1024],a,b;
    int n,i,j;
    bool f;
    while(~scanf("%d",&n))
    {
        if(n==0) break;
        memset(map,0,sizeof(map));
        f=0;
        for(i=0;i<n;i++)
        {
            scanf("%d%d",&a,&b);
            map[a][b]++;
        }
        for(i=0;i<1024;i++)
            {
                for(j=0;j<1024;j++)
                if(map[i][j]!=map[j][i]) {f=1;break;}
                if(f==1) break;
            }
        if(f) printf("NO
");
        else printf("YES
");
    }
    return 0;
}