int a[10001]={0};
int main(){
	int n,tag,i,j;
	scanf("%d",&n);
	while(1){
		scanf("%d%d",&i,&j);
		if(!(i||j)) break;
		a[i]=1;
	}
	for(int i=0;i<n;i++)
		if(!a[i]){
			printf("%d\n",i);
			return 0;
		}
	printf("NOT FOUND\n");
	/*
	for(int i=0;i<n;i++){
		tag=1;
		for(int j=0;j<n;j++){
			if(i==j) continue;
			if(a[i][j] || !(a[j][i]) ) {
				tag=0;
				break;
			}
		}
		if(tag==1){
			printf("%d\n",i);
			return 0;
		}
	}
	printf("NOT FOUND\n");*/


}