#include <bits/stdc++.h>
using namespace std;
#define PB push_back
#define F first
#define S second
typedef pair<int, int> pi;
int main() {
cin.tie(0);
ios::sync_with_stdio(0);
int N, M;
cin >> N >> M;
int B[M], P[M];
vector<int> dists[N];
map<int, int> m[N];
vector<bool> vis(N);
for (int i = 0; i < M; ++i) {
cin >> B[i] >> P[i];
dists[B[i]].PB(P[i]);
}
queue<pi> bfs({{B[0], P[0]}, {B[0], -P[0]}});
m[B[0]][P[0]] = 1;
while (!bfs.empty() && bfs.front().F != B[1]) {
auto tmp = bfs.front();
bfs.pop();
if (tmp.F + tmp.S < N && tmp.F + tmp.S >= 0 &&
m[tmp.F + tmp.S].find(abs(tmp.S)) == m[tmp.F + tmp.S].end()) {
m[tmp.F + tmp.S][abs(tmp.S)] = m[tmp.F][abs(tmp.S)] + 1;
bfs.push({tmp.F + tmp.S, tmp.S});
}
if (!vis[tmp.F])
for (auto i : dists[tmp.F]) {
m[tmp.F][i] = m[tmp.F][tmp.S];
if (tmp.F + i < N && m[tmp.F + i].find(i) == m[tmp.F + i].end()) {
m[tmp.F + i][i] = m[tmp.F][i] + 1;
bfs.push({tmp.F + i, i});
}
if (tmp.F - i >= 0 && m[tmp.F - i].find(i) == m[tmp.F - i].end()) {
m[tmp.F - i][i] = m[tmp.F][i] + 1;
bfs.push({tmp.F - i, -i});
}
}
vis[tmp.F] = 1;
}
cout << (bfs.empty() ? -1 : m[B[1]][abs(bfs.front().S)] - 1);
}
