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#include <iostream>#include <cstdio>#include <cstdlib>#include <cmath>#include <queue>#include <algorithm>#include <vector>#include <cstring>#include <stack>#include <cctype>#include <utility>   #include <map>#include <string>  #include <climits> #include <set>#include <string> #include <sstream>#include <utility>#include <ctime>using std::priority_queue;using std::vector;using std::swap;using std::stack;using std::sort;using std::max;using std::min;using std::pair;using std::map;using std::string;using std::cin;using std::cout;using std::set;using std::queue;using std::string;using std::istringstream;using std::make_pair;using std::greater;const int MAXN(500010);struct NODE{int a, b;friend bool operator != (const NODE &op1, const NODE &op2){return !(op1.a == op2.a && op1.b == op2.b);}friend bool operator < (const NODE &op1, const NODE &op2){return op1.a == op2.a? (op1.b < op2.b): (op1.a < op2.a);}};NODE arr1[MAXN], arr2[MAXN];int main(){int n;while(scanf("%d", &n), n){for(int i = 1; i <= n; ++i){scanf("%d%d", &arr1[i].a, &arr1[i].b);arr2[i].a = arr1[i].b;arr2[i].b = arr1[i].a;}sort(arr1+1, arr1+1+n);sort(arr2+1, arr2+1+n);int flag = 1;while(flag <= n){if(arr1[flag] != arr2[flag])break;++flag;}if(flag > n)printf("YES\n");elseprintf("NO\n");}return 0;}