bzoj 1620: [Usaco2008 Nov] Time Management time management [greedy]

Sort by s from large to small, reverse the time simulation work

#include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;
const int N=1005;
int n;
struct qwe
{
    int s,t;
}a[N];
bool cmp(const qwe &a,const qwe &b)
{
    return a.s>b.s;
}
int read()
{
    int r=0,f=1;
    char p=getchar();
    while(p>'9'||p<'0')
    {
        if(p=='-')
            f=-1;
        p=getchar();
    }
    while(p>='0'&&p<='9')
    {
        r=r*10+p-48;
        p=getchar();
    }
    return r*f;
}
int main()
{
    n=read();
    for(int i=1;i<=n;i++)
        a[i].t=read(),a[i].s=read();
    sort(a+1,a+1+n,cmp);
    int ti=1e9;
    for(int i=1;i<=n;i++)
    {
        ti=min(ti,a[i].s);
        ti=ti-a[i].t;
    }
    printf("%d\n",ti>=0?ti:-1);
    return 0;
}