[codevs1743]反转卡片

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <stack>
#include <vector>
#include <queue>
#include <cstring>
#include <string>
#include <map>
#include <set>
using namespace std;

const int BufferSize = 1 << 16;
char buffer[BufferSize], *Head, *Tail;
inline char Getchar() {
    if(Head == Tail) {
        int l = fread(buffer, 1, BufferSize, stdin);
        Tail = (Head = buffer) + l;
    }
    return *Head++;
}
int read() {
    int x = 0, f = 1; char c = Getchar();
    while(!isdigit(c)){ if(c == ‘-‘) f = -1; c = Getchar(); }
    while(isdigit(c)){ x = x * 10 + c - ‘0‘; c = Getchar(); }
    return x * f;
}

#define maxn 300010
#define maxq 100000
struct Node {
	int siz; bool rev;
	Node(): rev(0) {}
} ns[maxn];
int ToT, fa[maxn], ch[maxn][2], A[maxn];
void maintain(int o) {
	ns[o].siz = 1;
	for(int i = 0; i < 2; i++) if(ch[o][i])
		ns[o].siz += ns[ch[o][i]].siz;
	return ;
}
void build(int& o, int l, int r) {
	if(l > r) return ;
	int mid = l + r >> 1; o = A[mid];
	build(ch[o][0], l, mid - 1); build(ch[o][1], mid + 1, r);
	if(ch[o][0]) fa[ch[o][0]] = o;
	if(ch[o][1]) fa[ch[o][1]] = o;
	return maintain(o);
}
void pushdown(int o) {
	for(int i = 0; i < 2; i++) if(ch[o][i])
		ns[ch[o][i]].rev ^= ns[o].rev;
	if(ns[o].rev) swap(ch[o][0], ch[o][1]), ns[o].rev = 0;
	return ;
}
void rotate(int u) {
	int y = fa[u], z = fa[y], l = 0, r = 1;
	if(z) ch[z][ch[z][1]==y] = u;
	if(ch[y][1] == u) swap(l, r);
	fa[u] = z; fa[y] = u; fa[ch[u][r]] = y;
	ch[y][l] = ch[u][r]; ch[u][r] = y;
	maintain(y); maintain(u);
	return ;
}
int S[maxn], top;
void splay(int u) {
	int t = u; S[top = 1] = t;
	while(fa[t]) t = fa[t], S[++top] = t;
	while(top) pushdown(S[top--]);
	while(fa[u]) {
		int y = fa[u], z = fa[y];
		if(z) {
			if(ch[y][0] == u ^ ch[z][0] == y) rotate(u);
			else rotate(y);
		}
		rotate(u);
	}
	return ;
}
int split(int u) {
	if(!u) return splay(1), 1;
	splay(u);
	int tmp = ch[u][1];
	fa[tmp] = ch[u][1] = 0;
	return maintain(u), tmp;
}
int merge(int a, int b) {
	if(!a) return b;
	pushdown(a); while(ch[a][1]) a = ch[a][1], pushdown(a);
	splay(a);
	ch[a][1] = b; fa[b] = a;
	return maintain(a), a;
}
int Find(int o, int k) {
	pushdown(o);
	if(!o) return 0;
	int ls = ch[o][0] ? ns[ch[o][0]].siz : 0;
	if(k == ls + 1) return o;
	if(k > ls + 1) return Find(ch[o][1], k - ls - 1);
	return Find(ch[o][0], k);
}

int main() {
	int n = read();
	for(int i = 1; i <= n; i++) A[i] = read();
	if(A[1] == 1) return puts("0"), 0;
	
	int tmp = 0, i; build(tmp, 1, n);
	for(i = 1; i <= maxq; i++) {
		splay(1);
		int lrt = Find(1, Find(1, 1)), rrt = split(lrt);
		ns[lrt].rev ^= 1;
		lrt = merge(lrt, rrt);
		if(Find(lrt, 1) == 1) break;
	}
	
	printf("%d\n", i <= maxq ? i : -1);
	
	return 0;
}