#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <algorithm>

#define IOS ios::sync_with_stdio(false)
using namespace std;
#define inf (0x3f3f3f3f)
typedef long long int LL;


#include <iostream>
#include <sstream>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <string>
#include <bitset>
const int maxn = 1e3 + 20;
int t[maxn];
int val[maxn];
int T, H, n;
int mx[maxn];
int dp_max[maxn][20];
void init() {
    for (int i = 1; i <= n; ++i) {
        int tot = 0, useTime = 0;
        for (int j = i; j <= n; ++j) {
            useTime += t[j];
            if (useTime > T) break;
            tot += val[j];
        }
        mx[i] = tot;
    }
    for (int i = 1; i <= n; ++i) {
        dp_max[i][0] = mx[i];
    }
    for (int j = 1; j < 20; ++j) {
        for (int i = 1; i + (1 << j) - 1 <= n; ++i) {
            dp_max[i][j] = max(dp_max[i][j - 1], dp_max[i + (1 << (j - 1))][j - 1]);
        }
    }
}
int ask(int be, int en) {
    if (be >  en) return 0;
    int k = (int)log2(en - be + 1.0);
    return max(dp_max[be][k], dp_max[en - (1 << k) + 1][k]);
}
void work() {
    scanf("%d%d%d", &T, &H, &n);
    int hurt = 0;
    for (int i = 1; i <= n; ++i) {
        scanf("%d%d", &t[i], &val[i]);
        hurt += val[i];
    }
    init();
    int mxsave = 0, ans = 0;
    for (int i = 1; i <= n; ++i) {
        int useTime = 0;
        int tot = 0;
        for (int j = i; j <= n; ++j) {
            useTime += t[j];
            if (useTime > 2 * T) {
                break;
            }
            tot += val[j];
        }
        mxsave = max(mxsave, tot);
    }
    ans = H - (hurt - mxsave);
    for (int i = 1; i <= n; ++i) {
        int tot = 0, useTime = 0;
        for (int j = i; j <= n; ++j) {
            useTime += t[j];
            if (useTime > T) break;
            tot += val[j];
            int res = ask(j + 1, n);
            mxsave = max(mxsave, tot + res);
        }
    }
    ans = max(ans, H - (hurt - mxsave));
    ans = max(ans, 0);
    cout << ans << endl;
}

int main() {
#ifdef local
    freopen("data.txt", "r", stdin);
//    freopen("data.txt", "w", stdout);
#endif
    work();
    return 0;
}