#include <bits/stdc++.h>
using namespace std;
const int N = 10;
const int inf = 1e9 + 9;
int n, m, target, b[N], p[N], dp[N][N][2][1 << N][8];
vector<int> g[N];
int sol(int i, int j, int dir, int vis, int dn) {
if (vis & (1 << i)) {
return inf;
}
if (i == target) {
return 0;
}
int& ret = dp[i][j][dir][vis][dn];
if (ret != -1) {
return ret;
}
ret = inf;
int new_v = vis | (1 << i);
if (dir) {
if (i + p[j] < n) {
ret = min(ret, 1 + sol(i + p[j], j, dir, new_v, dn));
}
} else {
if (i - p[j] >= 0) {
ret = min(ret, 1 + sol(i - p[j], j, dir, new_v, dn));
}
}
for (int new_j : g[i]) {
if (new_j != j && !(dn & (1 << new_j))) {
ret = min(ret, sol(i, new_j, 1, 0, dn | (1 << j)));
ret = min(ret, sol(i, new_j, 0, 0, dn | (1 << j)));
}
}
return ret;
}
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
cin >> n >> m;
for (int i = 0; i < m; i++) {
cin >> b[i] >> p[i];
g[b[i]].push_back(i);
}
target = b[1];
memset(dp, -1, sizeof dp);
int ans = sol(0, 0, 1, 0, 1);
if (ans >= inf) {
cout << -1 << '\n';
} else {
cout << ans << '\n';
}
return 0;
}
