所以......这为啥RE啊

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

int n;
int x,y;
int a[100005];
int op;

int main(){
	scanf("%d",&n);
	for(long long i = 1; i <= n; ++i){
		scanf("%d%d",&x,&y);
		a[++op] = x - y;
		a[++op] = y;
	}
	sort(a + 1,a + op + 1);
	printf("%d",a[op] + a[op - 1]);
	return 0;
}