求好心人改代码

大概是理清楚了，但是check不会写了，求大佬帮忙

#include<cstdio>
using namespace std;
struct ddd{
    long long x,s;
}a[500010];
int n,d;
long long k;
bool check(int mid){
    long long sc;//score前两位
    
    return sc>=k;
}
int main(){
    int ans=-1;
    scanf("%lld %lld %lld",&n,&d,&k);
    for(int i=0;i<n;i++){
        scanf("%lld %lld",&a[i].x,&a[i].s);
    }
    int l=0,r=a[n-1].x;
    while(l<=r){
        int mid=(l+r)/2;
        if(check(mid)){
            ans=mid;
            r=mid-1;
        }else{
            l=mid+1;
        }
    }
    printf("%d",ans);
}