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| #include <iostream>
#include <vector>
#include <string>
#include <cstring>
#include <algorithm>
#include <sstream>
#include <map>
#include <set>
#include <queue>
#define REP(i,k,n) for(int i=k;i<n;i++)
#define rep(i,n) for(int i=0;i<n;i++)
#define INF 1LL<<60
#define pb push_back
#define mp make_pair
using namespace std;
typedef long long ll;
typedef pair<int,int> P;
typedef pair<P, bool> PI;
int w, h, k;
vector<P> X[200005], Y[200005];
vector<P> xy;
ll d[200005][2];
// PI f(int a, int b, int c) {
// return mp(mp(a, b), c);
// }
void dijkstra() {
rep(i, 200005) {
d[i][0] = INF;
d[i][1] = INF;
}
// cost id (0 or 1)
priority_queue<PI, vector<PI>, greater<PI> > que;
rep(i, X[1].size()) {
int y = X[1][i].first;
int to = X[1][i].second;
d[to][1] = y;
// que.push(f(y, to, 1));
que.push(mp(mp(y, to), 1));
}
while(que.size()) {
PI p = que.top();
que.pop();
int c = p.first.first;
int v = p.first.second;
int state = p.second;
if(d[v][state] < c) continue;
int y = xy[v].first;
int x = xy[v].second;
if(state == 0) {
rep(i, X[x].size()) {
int ny = X[x][i].first;
int to = X[x][i].second;
if(ny == y) continue;
if(d[to][1] > d[v][state] + abs(ny - y) + 1) {
d[to][1] = d[v][state] + abs(ny- y) + 1;
// que.push(f(d[to][1], to, 1));
que.push(mp(mp(d[to][1], to), 1));
}
}
} else {
rep(i, Y[y].size()) {
int nx = Y[y][i].first;
int to = Y[y][i].second;
if(nx == x) continue;
if(d[to][0] > d[v][state] + abs(nx - x) + 1) {
d[to][0] = d[v][state] + abs(nx - x) + 1;
// que.push(f(d[to][0], to, 0));
que.push(mp(mp(d[to][0], to), 0));
}
}
}
}
}
int main() {
cin >> w >> h >> k;
xy.resize(k);
rep(i, k) {
int x, y;
cin >> x >> y;
xy[i] = mp(y, x);
X[x].push_back(mp(y, i));
Y[y].push_back(mp(x, i));
}
dijkstra();
ll ans = INF;
rep(i, k) {
int y = xy[i].first;
int x = xy[i].second;
if(y == h) {
ans = min(ans, d[i][1] + abs(w - x));
}
if(x == w) {
ans = min(ans, d[i][0] + abs(h - y));
}
}
if(ans == INF) cout << -1 << endl;
else cout << ans << endl;
return 0;
}
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