#include <bits/stdc++.h>
using namespace std;
#define st first
#define nd second
const long long INF = 4e18;
pair<long long, long long> P[303];
long long a[303], S[707], E[707], X[3][707], dp[303][303], F[2][303][303], L, U;
int dq[3][707], n;
pair<long long, int> V[303];
int f[303], g[303];
long long solve(long long len){
int sz = 0;
for(int i = 1, j = 1; i <= n || j <= n;){
long long s = min(a[i], a[j] + len);
while(a[i] == s || a[j] + len == s){
if(a[i] == s) i++;
if(a[j] + len == s) j++;
}
if(j > n) break;
long long e = min(a[i], a[j] + len);
assert(j < i);
sz++;
S[sz] = s, E[sz] = e;
X[0][sz] = F[0][j][i - 1], X[1][sz] = F[1][j][i - 1], X[2][sz] = dp[j][i - 1];
}
long long ret = INF;
int pl[3] ={1, 1, 1}, pr[3] ={1, 1, 1};
for(int i = 1, j = 1; i <= sz; i++){
for(int z = 0; z < 3; z++) while(pl[z] < pr[z] && dq[z][pl[z]] < i) pl[z]++;
while(j <= sz && E[j - 1] - S[i] < L){
for(int z = 0; z < 3; z++){
while(pl[z] < pr[z] && X[z][dq[z][pr[z] - 1]] <= X[z][j]) pr[z]--;
dq[z][pr[z]] = j;
pr[z]++;
}
j++;
}
if(j > sz && E[j - 1] - S[i] < L) break;
for(int z = 0; z < 3; z++) assert(pl[z] < pr[z]);
ret = min(ret, max(X[0][dq[0][pl[0]]] + X[1][dq[1][pl[1]]], X[2][dq[2][pl[2]]]));
}
return ret + len - 1;
}
int main(){
if(fopen("csort.inp", "r")){
freopen("csort.inp", "r", stdin);
freopen("csort.out", "w", stdout);
}
cin.tie(0)->sync_with_stdio(0);
cout.tie(0)->sync_with_stdio(0);
cin >> L >> U;
cin >> n;
for(int i = 1; i <= n; i++) cin >> P[i].first >> P[i].second;
sort(P + 1, P + n + 1);
for(int i = 1; i <= n; i++) a[i] = P[i].first;
for(int i = n; i > 0; i--){
int sz = 0;
V[sz++] ={0, 0};
for(int j = n; j >= i; j--){
V[sz++] ={P[j].second, j};
}
V[sz++] ={U + 1, n + 1};
sort(V, V + sz);
for(int i = 0; i < sz - 1; i++) f[V[i].second] = V[i + 1].second, g[V[i + 1].second] = V[i].second;
for(int j = 1; j < sz; j++) dp[i][n] = max(dp[i][n], V[j].first - V[j - 1].first - 1);
F[0][i][n] = P[f[0]].second - 1;
F[1][i][n] = U - P[g[n + 1]].second;
for(int j = n - 1; j >= i; j--){
int ff = f[j + 1], gg = g[j + 1];
g[ff] = gg;
f[gg] = ff;
dp[i][j] = max(dp[i][j + 1], P[ff].second - P[gg].second - 1);
F[0][i][j] = P[f[0]].second - 1;
F[1][i][j] = U - P[g[n + 1]].second;
}
}
long long mn = 0;
for(int i = 2; i <= n; i++) mn = max(mn, a[i] - a[i - 1]);
mn = max(mn,(L - a[n]) + a[1]);
a[n + 1] = INF;
long long ans = INF;
vector<long long> C ={L};
for(int i = 1; i <= n; i++){
for(int j = i + 1; j <= n; j++){
if(a[j] - a[i] >= mn) C.push_back(a[j] - a[i]);
if(L - a[j] + a[i] >= mn) C.push_back(L - a[j] + a[i]);
if(L + a[j] - a[i] >= mn) C.push_back(L + a[j] - a[i]);
}
}
sort(C.begin(), C.end());
C.erase(unique(C.begin(), C.end()), C.end());
for(long long x : C) ans = min(ans, solve(x));
cout << ans << '\n';
}
