题解来自洛谷网友

首先，先证明一个结论。 代码实现：

#include <bits/stdc++.h>
#define int long long
using namespace std;

int n, k;
int d, m;
int t[300005];
int maxd = 0, ans = 0, uk;

signed main(){
    scanf("%lld%lld", &n, &k);
    for (int i = 1; i <= n; i++){
        scanf("%lld%lld", &d, &m);
        t[d]+=m;maxd = max(maxd, d);
    }
    for (int i = 0; i <= maxd+1; i++){
        if (i == 0){
            if (t[i] <= k) ans+=t[i], t[i] = 0;
            else{
                ans+=k, t[i]-=k;
            }
        }else{
            if (t[i-1] <= k) ans+=t[i-1], uk = t[i-1], t[i-1]=0;
            else {
                ans+=k, uk = k, t[i-1]-=k;continue ;
            }
            if (t[i]<=k-uk) ans+=t[i], t[i] = 0;
            else ans+=k-uk, t[i]-=(k-uk);
        }
    }
    printf("%lld", ans);
    return 0;
}