求助，为什么答案减一才能过

#include<bits/stdc++.h>
using namespace std;
const int N=1.5e5+10,M=3e6+5;
inline int read(){
 	int x=0;bool f=0;
	char s=getchar();
	while(!isdigit(s)){if(s=='-') f=1;s=getchar();}
	while(isdigit(s)) x=(x<<3)+(x<<1)+s-'0',s=getchar();
	return f?-x:x;
}
int n,maxn,dp[M],j,ans;
struct node{
    int x,y,s;
}a[N];
inline bool cmp(node a,node b){
	return a.x!=b.x?a.x<b.x:a.y<b.y;
}
signed main(){
    n=read();
    for(int i=1;i<=n;i++){
		a[i].x=read(),a[i].y=read(),a[i].s=a[i].y-a[i].x+1;
		maxn=max(maxn,a[i].y);
    }
    sort(a+1,a+n+1,cmp);j=1;
    for(int i=0;i<=maxn;i++){
        dp[i]=max(dp[i],dp[i-1]);
        while (a[j].x==i&&j<=n){
            dp[a[j].y]=max(dp[a[j].y],dp[a[j].x-1]+a[j].s);
            j++;
        }
        ans=max(ans,dp[i]);
    }
    printf("%d",ans-1);
    return 0;
}