为什么5个点全RE？

#include<bits/stdc++.h>
using namespace std;
struct Node{
	string s;
	int y;
	int m;
	int d;
	int num;
}a[105];
bool cmp(Node a,Node b){
	if(a.y!=b.y) return a.y<b.y;
	if(a.m!=b.m) return a.m<b.m;
	if(a.d!=b.d) return a.d<b.d;
	return a.num<b.num;
}
int main(){
	int n;
	for(int i=1;i<=n;i++){
		cin >> a[i].s >> a[i].y >> a[i].m >> a[i].d;
		a[i].num=i;
	}
	sort(a+1,a+n+1,cmp);
	for(int i=1;i<=n;i++){
		cout << a[i].s << endl;
	}
	return 0;
}