Rest Stop

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

int l, n, f, b;
ll rest[1000001];
int mx[1000001];
ll ans;

int main(){
    freopen("reststops.in", "r", stdin);
    freopen("reststops.out", "w", stdout);

    cin >> l >> n >> f >> b;
    int cover = f-b;
    for(int i=0; i<n; i++){
        ll p, t;
        cin >> p >> t;
        rest[p] = t;
    }
    mx[l] = l;
    for(int i=l-1; i>=1; i--){
        if(rest[mx[i+1]]<rest[i]){
            mx[i] = i;
        }else{
            mx[i] = mx[i+1];
        }
    }
    //for(int i=1; i<=l; i++){
        //cout << i << ", " << mx[i] << ", " << rest[mx[i]] << '\n';
    //}
    int pos = mx[1];
    int prev = 0;
    while(pos<=l){
        ans+=(cover*rest[pos]*(pos-prev));
        if(pos>=l) break;
        prev = pos;
        pos = mx[pos+1];
    }
    cout << ans << '\n';

    return 0;
}