这道题

#include<cmath>
#include<cstdio>
#include<iostream>
using namespace std;
const int N=20005;
int l[N],r[N],n,f[N][5]; 
int main(){
	cin>>n;
	for(int i=1;i<=n;i++)
		cin>>l[i]>>r[i];
	f[1][0]=r[1]-1+r[1]-l[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],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],f[i-1][0]+abs(l[i-1]-l[i])+r[i]-l[i])+1;
	f[n][2]=min(f[n][0],f[n][1]);
	cout<<f[n][2]<<endl; 
	return 0;
}

刚编完，编译时line15说“[Error] name lookup of 'i' changed for ISO 'for' scoping [-fpermissive]” 和 “[Note] (if you use '-fpermissive' G++ will accept your code)”