/*
5 3
0 2
1 1
4 1
*/
#include<bits/stdc++.h>
using namespace std;
#define INF 1000000000
#define MAXN 2000
#define MAXP 2000
int n, m, startPosition, startPower, endPosition, dist[MAXN + 3][MAXP + 3];
vector<int> dogesAtPosition[MAXN + 3];
int main() {
cin >> n >> m;
for (int i = 0; i < m; i++) {
int b, p;
cin >> b >> p;
if (i == 0) {
startPosition = b;
startPower = p;
}
if (i == 1) {
endPosition = b;
}
dogesAtPosition[b].push_back(p);
}
for (int i = 0; i < n; i++) {
sort(dogesAtPosition[i].begin(), dogesAtPosition[i].end());
dogesAtPosition[i].erase(unique(dogesAtPosition[i].begin(), dogesAtPosition[i].end()), dogesAtPosition[i].end());
}
for (int i = 0; i < n; i++) {
for (int p = 1; p <= MAXP; p++) {
dist[i][p] = INF;
}
}
dist[startPosition][startPower] = 0;
priority_queue<pair<int, pair<int, int> > > pq;
pq.push({-dist[startPosition][startPosition], {startPosition, startPower}});
while (!pq.empty()) {
auto [d, state] = pq.top(); pq.pop();
auto [position, power] = state;
d *= -1; // Hacky
if (d > dist[position][power]) {
continue;
}
// Pass to other doge
for (auto p: dogesAtPosition[position]) {
if (dist[position][p] > dist[position][power]) {
dist[position][p] = dist[position][power];
pq.push({-dist[position][p], {position, p}});
}
}
// Jump
for (int direction = -1; direction <= 1; direction += 2) {
int newPosition = position + direction * power;
if (newPosition >= 0 && newPosition < n && dist[newPosition][power] > dist[position][power] + 1) {
dist[newPosition][power] = dist[position][power] + 1;
pq.push({-dist[newPosition][power], {newPosition, power}});
}
}
}
int ans = INF;
for (int p = 1; p <= MAXP; p++) {
ans = min(ans, dist[endPosition][p]);
}
if (ans < INF) {
cout << ans << endl;
} else {
cout << -1 << endl;
}
}
