这个 不会看题解后想自己做一遍，但是#2之后全WA，这是为什么啊

#include<cmath>
#include<cstdio>
#include<iostream>
using namespace std;
inline int read(){
    int s = 0, w = 1;
    char ch = getchar();
    while(ch < '0' || ch > '9'){if(ch == '-')w = -1;ch = getchar();}
    while(ch >= '0' && ch <= '9') s = s * 10 + ch - '0',ch = getchar();
    return s * w;
}
const int N=20005;
int l[N],r[N],n,f[N][2]; 
int main(){
	n=read();
	for(int i=1;i<=n;i++){
		l[i]=read();
		r[i]=read();
	}
	f[1][0]=r[1]+r[1]-l[1]-1;
	f[1][1]=r[1]-1;
	for(int i=2;i<=n;i++){
		f[i][0]=min(f[i-1][0]+abs(l[i-1]-r[i])+r[i]-l[i]+1,f[i-1][1]+abs(r[i-1]-l[i])+r[i]-l[i]+1);
		f[i][1]=min(f[i-1][1]+abs(r[i-1]-l[i])+r[i]-l[i]+1,f[i-1][0]+abs(l[i-1]-l[i])+r[i]-l[i]+1);
	}
	cout<<min(f[n][0]+n-l[n],f[n][1]+n-r[n])<<endl; 
	return 0;
}