3.cpp

#include<iostream>
#include<algorithm>

using namespace std;

typedef long long ll;

ll C,S,T,N;
ll solve(ll,ll,ll ,ll);
ll work[2][101];
short int dp[5000][100001];

int main(){
	int p = 0;
	cin >> C >> S >> T;
	cin >> N;

	for(ll i=0;i<5000;i++)for(long j=0;j<100001;j++)dp[i][j] = -1;

	for(int i=0;i<N;i++){
		cin >> work[0][i] >> work[1][i];
	}
	cout << solve(0,S,C,0) << endl;
	return 0;
}

ll solve(ll month,ll i_s,ll i_c,ll i_n){
	if(dp[month][i_s] != -1)return dp[month][i_s];
	if(i_s >= T){
		return dp[month][i_s] = month;
	}
	else{
		ll res=-1,rec,t = i_s+i_c,tm = i_s+i_c+work[1][i_n]-work[0][i_n];
		if(i_s >= work[0][i_n] && i_n < N)res = solve(month+1,tm,work[1][i_n]+i_c,i_n+1);
		rec = solve(month+1,t,i_c,i_n);
		
		if(res != -1){
			if(rec < res)t = tm;
			rec = min(res,rec);
		}
		return dp[rec][t] = rec;		
	}
}