C. Songs Compression（简单贪心）

#include<iostream>
#include<algorithm>
using namespace std;
#define LL long long
const int maxn=1e5+10;
struct node{int x, y;}a[maxn];
bool cmp(node a, node b){
    return a.x-a.y>b.x-b.y;
}
LL n, m, len, ans, ans1;

int main(){
    cin>>n>>m;
    for(int i=0;i<n;++i){
        cin>>a[i].x>>a[i].y;
        ans+=a[i].x;        ans1+=a[i].y;
    }
    if(ans<=m){cout<<0<<endl; return 0;}
    if(ans1>m){cout<<-1<<endl;  return 0;}
    else if(ans1==m){cout<<n<<endl; return 0;}
    sort(a, a+n, cmp);
    for(int i=0;i<n;++i){
        ans=ans-a[i].x+a[i].y;     ++len;
        if(ans<=m)break;
    }
    cout<<len<<endl;
    return 0;
}