易错点：

• 需要熟练掌握斜率优化DP的原理与实现方法.
• 二分时需要仔细判定边界条件.

#include<cstdio>
#include<iostream>
#define ll long long
using namespace std;
const int MAXN=3e5+10;
ll f[MAXN],sumT[MAXN],sumC[MAXN];
int q[MAXN],l=1,r=1;
int binary_search(int k){
    int L=l,R=r;
    while(L<R){
        int mid=(L+R)>>1;
        if(f[q[mid+1]]-f[q[mid]]<k*(sumC[q[mid+1]]-sumC[q[mid]]))L=mid+1;
        else R=mid;;
    }
    return q[L];
}
int main(){
    int n,s;
    scanf("%d%d",&n,&s);
    for(int i=1;i<=n;i++){
        int t,c;
        scanf("%d%d",&t,&c);
        sumT[i]=sumT[i-1]+t;
        sumC[i]=sumC[i-1]+c;
    }
    for(int i=1;i<=n;i++){
        int p=binary_search(s+sumT[i]);
        f[i]=f[p]+sumT[i]*sumC[i]-sumT[i]*sumC[p]+s*sumC[n]-s*sumC[p];
        while(l<r&&(f[i]-f[q[r]])*(sumC[q[r]]-sumC[q[r-1]])<=(f[q[r]]-f[q[r-1]])*(sumC[i]-sumC[q[r]]))r--;
        q[++r]=i;
    }
    printf("%lld\n",f[n]);
    return 0;
}