为什莫有一个点老是访问无效内存

#include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;
long long sum,sumj,n;
struct re
{
    long long s,j;
};
struct re a[500009];
int myway(struct re a,struct re b)
{
    if(a.j==b.j)
       return a.s>=b.s;
     else  
    return a.j<b.j;
}
int main()
{
    cin>>sum>>n;
    for(int i=1;i<=n;i++)
      scanf("%d%d",&a[i].j,&a[i].s);
    sort(a+1,a+n+1,myway); 
    for(int i=1;i<=n;i++)
    if(sum==0) break;
     else
       if(sum>=a[i].s)
        {
         sum-=a[i].s;
         sumj+=a[i].s*a[i].j;
        } 
         else
           if(sum<a[i].s)
            {
              sumj+=sum*a[i].j;
              sum=0;
            }
    cout<<sumj;
    return 0;        
}
求大神犇指点啊