#include<bits/stdc++.h>
#include<atcoder/all>
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;
using namespace atcoder;
typedef long long ll;
typedef vector<int> vi;
typedef vector<long long> vl;
typedef vector<vector<int>> vvi;
typedef vector<vector<long long>> vvl;
typedef long double ld;
typedef pair<int, int> P;

ostream& operator<<(ostream& os, const modint& a) {os << a.val(); return os;}
template <int m> ostream& operator<<(ostream& os, const static_modint<m>& a) {os << a.val(); return os;}
template<typename T> istream& operator>>(istream& is, vector<T>& v){int n = v.size(); assert(n > 0); rep(i, n) is >> v[i]; return is;}
template<typename U, typename T> ostream& operator<<(ostream& os, const pair<U, T>& p){os << p.first << ' ' << p.second; return os;}
template<typename T> ostream& operator<<(ostream& os, const vector<T>& v){int n = v.size(); rep(i, n) os << v[i] << (i == n - 1 ? "\n" : " "); return os;}
template <typename T> ostream& operator<<(ostream& os, const vector<vector<T>>& v){int n = v.size(); rep(i, n) os << v[i] << (i == n - 1 ? "\n" : ""); return os;}


int main(){
	int h, w;
	cin >> h >> w;
	vvi a(h, vi(h + w));
	rep(i, w){
		int x, y;
		cin >> x >> y;
		x--; y--;
		a[x][y]++;
	}
	int cnt = 0;
	long long ans = 0;
	rep(y, h + w) for(int x = h - 1; x >= 0; x--){ 
		if(a[x][y] > 0){
			long long dist = 0;
			if(y >= cnt and y <= cnt + x) dist = x;
			else dist = x + min(abs(y - cnt), abs(y - (cnt + x)));
			ans += dist;
			cnt++;
		}
	}
	cout << ans << "\n";
	return 0;
}