#include <bits/stdc++.h>

#define CHOOSE(a) CHOOSE2 a
#define CHOOSE2(a0, a1, a2, a3, a4, x, ...) x
#define dump_1(x1) cerr << #x1 << ": " << x1 << endl
#define dump_2(x1, x2) \
  cerr << #x1 << ": " << x1 << ", " #x2 << ": " << x2 << endl
#define dump_3(x1, x2, x3)                                                \
  cerr << #x1 << ": " << x1 << ", " #x2 << ": " << x2 << ", " #x3 << ": " \
       << x3 << endl
#define dump_4(x1, x2, x3, x4)                                            \
  cerr << #x1 << ": " << x1 << ", " #x2 << ": " << x2 << ", " #x3 << ": " \
       << x3 << ", " #x4 << ": " << x4 << endl
#define dump_5(x1, x2, x3, x4, x5)                                        \
  cerr << #x1 << ": " << x1 << ", " #x2 << ": " << x2 << ", " #x3 << ": " \
       << x3 << ", " #x4 << ": " << x4 << ", " #x5 << ": " << x5 << endl
#define dump(...) \
  CHOOSE((__VA_ARGS__, dump_5, dump_4, dump_3, dump_2, dump_1, ~))(__VA_ARGS__)
#define check(s) cerr << s << endl

#define rep(i, n) for (int i = 0; i < n; i++)
#define repr(i, n) for (int i = n; i >= 0; i--)
#define FOR(i, m, n) for (int i = m; i < n; i++)
#define all(x) (x).begin(), (x).end()
#define sz(x) ((int)(x).size())
#define unique(v) v.erase(unique(v.begin(), v.end()), v.end());
#define chmax(x, y) x = max(x, y)
#define chmin(x, y) x = min(x, y)

using namespace std;

using ll = long long;

vector<int> dx = {0, 1, 0, -1};
vector<int> dy = {1, 0, -1, 0};

const ll LINF = 2e18;
const int INF = 1e9;

void solve(ll N, std::vector<ll> A, std::vector<ll> B) {
  ll sum = 0;

  rep(i, N) {
    ll p = (A.at(i) + 1) * A.at(i) / 2;
    ll q = (B.at(i) + 1) * B.at(i) / 2;
    sum += q - p + A.at(i);
  }
  cout << sum << endl;
}

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
  ll N;
  scanf("%lld", &N);
  std::vector<ll> A(N);
  std::vector<ll> B(N);
  for (int i = 0; i < N; i++) {
    scanf("%lld", &A[i]);
    scanf("%lld", &B[i]);
  }
  solve(N, std::move(A), std::move(B));
  return 0;
}

#include <bits/stdc++.h>
template<class T> inline bool chmin(T&a, T b){if(a > b){a = b; return true;}else{return false;}}
template<class T> inline bool chmax(T&a, T b){if(a < b){a = b; return true;}else{return false;}}
#define ll long long
#define double long double
#define rep(i,n) for(int i=0;i<(n);i++)
#define REP(i,n) for(int i=1;i<=(n);i++)
#define mod (ll)(1e9+7)
#define inf (ll)(3e18+7)
#define eps (double)(1e-9)
#define pi (double) acos(-1)
#define P pair<int,int>
#define PiP pair<int,pair<int,int>>
#define all(x) x.begin(),x.end()
#define rall(x) x.rbegin(),x.rend()
using namespace std;

int main() {
    ll n, m;
    cin >> n >> m;
    vector<ll> a(m);
    rep(i, m)cin >> a[i];
    sort(all(a));
    a.insert(a.begin(), 0);
    a.push_back(n+1);
    vector<ll> b;
    rep(i, (int)a.size()-1){
        if(a[i+1] - a[i] <= 1)continue;
        b.push_back(a[i+1]-a[i]-1);
    }
    if(b.size() == 0){
        cout << 0 << endl;
        return 0;
    }
    ll ans = 0;
    ll g = *min_element(all(b));
    rep(i, b.size())ans += (b[i]+g-1)/g;
    cout << ans << endl;
}   