8VC Venture Cup 2016 - Final Round (Div2) E

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#define LL long long

using namespace std;

const int MAXN = 200050;

int d, n, m;


int gp[MAXN], gpz[MAXN];
int fl[MAXN];	

struct gas{
	int pos, price;
}g[MAXN];

bool cmp(gas a, gas b){
	if(a.pos < b.pos) return true;
	return false;
}


int main(){
	scanf("%d%d%d", &d, &n, &m);
	
	gp[0] = 0, gpz[0] = 1e7;
	for(int i = 1; i <= m; i++){
		scanf("%d%d", &g[i].pos, &g[i].price);
	}
	
	sort(g + 1, g + 1 + m, cmp);
	
	for(int i = 1; i <= m; i++){
		gp[i] = g[i].pos, gpz[i] = g[i].price;
	}
	
	gp[m + 1] = d, gpz[m + 1] = 0;
	
	/*
	for(int i = 0; i<= m + 1; i++)
		cout <<gp[i] <<" "<< gpz[i] << endl;
	cout << endl;
	*/
	int mindist = -1;
	
	for(int i = 0; i <= m; i++){
		mindist = max(mindist, gp[i + 1] - gp[i]);
	}
	
//	cout << mindist << endl;
	
	if(mindist > n){
		puts("-1");
		return 0;
	}
	
	if(d <= n){
		puts("0");
		return 0;
	}
	
	for(int i = m; i >= 0; i--){
		if(gpz[i + 1] <= gpz[i]){
			fl[i] = i + 1;
		}
		else {
			int tmp = i + 1;
			while(gpz[fl[tmp]] > gpz[i]){
				tmp = fl[tmp];
			}
			fl[i] = fl[tmp];
		}
	}
	/*
	for(int i = 0; i<= m; i++){
		cout << fl[i] <<" ";
	}
	cout << endl;
	
	*/
	
	LL ans = 0;
	int liter = n;
	
	for(int l = 0; l < m+ 1; ){
		if(gp[fl[l]] - gp[l] > n){
			ans += (LL)(n - liter) * gpz[l];
			int tmp = l + 1;
			while(gp[fl[tmp]] - gp[l] <= n){
				tmp = fl[tmp];
			}
			liter = n - (gp[tmp] - gp[l]);
			l = tmp;
		}
		else{
			int tmp = fl[l];
			if(tmp == m + 1){
				ans += (LL)(d - gp[l] - liter) * gpz[l];
				break;
			}
			else{
				if(gp[tmp] - gp[l] <= liter){
					liter -= gp[tmp] - gp[l];
				}
				else{
					ans += (LL)(gp[tmp] - gp[l] - liter)*gpz[l];
					liter = 0;
				}
				l = tmp;
			}
		}
	//	cout << l << endl;
	//	system("pause");
	}
	cout << ans << endl;
	
}