// ~~ icebear love attttt ~~
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> ii;
typedef pair<int, ii> iii;
template<class T>
bool minimize(T &a, const T &b) {
if (a > b) return a = b, true;
return false;
}
template<class T>
bool maximize(T &a, const T &b) {
if (a < b) return a = b, true;
return false;
}
#define FOR(i,a,b) for(int i=(a); i<=(b); ++i)
#define FORR(i,a,b) for(int i=(a); i>=(b); --i)
#define REP(i, n) for(int i=0; i<(n); ++i)
#define RED(i, n) for(int i=(n)-1; i>=0; --i)
#define MASK(i) (1LL << (i))
#define BIT(S, i) (((S) >> (i)) & 1)
#define mp make_pair
#define pb push_back
#define fi first
#define se second
#define all(x) x.begin(), x.end()
#define task "icebearat"
#define index loveat
const int MOD = 1e9 + 7;
const int inf = 1e9 + 27092008;
const ll INF = 1e18 + 27092008;
const int N = 30000 + 5;
const int THRESHOLD = 500;
int n, m;
vector<ii> G[N];
int dist[N][THRESHOLD + 5], index[N], far[N];
vector<int> doges[N];
bool active[N];
struct state {
int dist, pos, jump;
bool operator > (const state &other) const {
return dist > other.dist;
}
};
priority_queue<state, vector<state>, greater<state>> Q;
#define HEAVY_FAR(x) ((x) > THRESHOLD)
void init(void) {
cin >> n >> m;
REP(i, m) {
cin >> index[i] >> far[i];
if (HEAVY_FAR(far[i])) {
for(int num = 0; index[i] + num * far[i] < n; num++)
G[index[i]].emplace_back(index[i] + num * far[i], num);
for(int num = 0; index[i] - num * far[i] >= 0; num--)
G[index[i]].emplace_back(index[i] - num * far[i], num);
far[i] = THRESHOLD + 1;
} else doges[index[i]].pb(far[i]);
}
}
void process(void) {
memset(dist, 0x3f, sizeof dist);
assert(far[0] <= THRESHOLD + 1);
dist[index[0]][far[0]] = 0;
Q.push({0, index[0], far[0]});
while(!Q.empty()) {
int d = Q.top().dist;
int pos = Q.top().pos;
int jump = Q.top().jump;
Q.pop();
assert(pos < n);
assert(jump <= THRESHOLD + 1);
if (dist[pos][jump] != d) continue;
if (pos == index[1]) {
cout << d;
exit(0);
}
if (!HEAVY_FAR(jump)) {
if (pos + jump < n && minimize(dist[pos + jump][jump], d + 1))
Q.push({d + 1, pos + jump, jump});
if (pos - jump >= 0 && minimize(dist[pos - jump][jump], d + 1))
Q.push({d + 1, pos - jump, jump});
}
if (!active[pos]) {
for(int dogeJump : doges[pos]) {
if (minimize(dist[pos][dogeJump], d))
Q.push({d, pos, dogeJump});
}
for(ii x : G[pos]) {
int v, w; tie(v, w) = x;
int nxt = THRESHOLD + 1;
if (minimize(dist[v][nxt], d + w))
Q.push({dist[v][nxt], v, nxt});
}
active[pos] = true;
}
}
cout << -1;
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
if (fopen(task".inp", "r")) {
freopen(task".inp", "r", stdin);
freopen(task".out", "w", stdout);
}
int tc = 1;
// cin >> tc;
while(tc--) {
init();
process();
}
return 0;
}
