最后输出这个-1是为什么？自己写的忘记原因了

#include <bits/stdc++.h>
using namespace std;

int main()
{
    int n,s,a,b,t = 1,sum = 0;
    int x,y;
    int k[5001] = {0};
    cin >> n >> s >> a >> b;
    for(int i = 1; i <= n; i++)
    {
        cin >> x >> y;
        if(x <= a + b)
        {
            k[t] = y;
            t++;
        }
    }
    for(int i = t - 1; i >= 1; i--)
    {
        for(int j = 1; j <= i; j++)
        {
            if(k[j] > k[j + 1])
            {
                swap(k[j],k[j + 1]);
            }
        }
    }
    for(int i = 1; i <= t; i++)
    {
        if(s - k[i] >= 0)
        {
            s = s - k[i];
            sum++;
        }
    }   
    cout << sum - 1;
    return 0;
}