我同学拿n方草过去了 https://www.luogu.com.cn/record/176139624

#include<bits/stdc++.h>
using namespace std;
#define int long long
int n,s,t[500010],f[500010],dp[500010];
signed main(){
    cin>>n>>s;
    memset(dp,0x3f,sizeof(dp));
    dp[0]=0;
    for(int i=1;i<=n;i++) cin>>t[i]>>f[i];
    for(int i=1;i<=n;i++) t[i]+=t[i-1],f[i]+=f[i-1];
    for(int i=1;i<=n;i++){
        for(int j=1;j<=i;j++){
            dp[i]=min(dp[i],dp[j-1]+t[i]*(f[i]-f[j-1])+s*(f[n]-f[j-1]));
        }
    }
    cout<<dp[n];
    return 0;
}