CE

#include <iostream>
#define int long long
#include <cstdio>
using namespace std;

int n, x, a[100010];
int ans = 0;

signed main()
{
	ios::sync_with_stdio(NULL);
	cin >> n >> x;
	for (int i = 1; i <= n; i++){
		cin >> a[i];
	}
	if (a[1]>x){
		ans += (a[1] - x);
		a[1] = x;
	}
	for (int i = 2; i <= n; i++){
		if (a[i]+a[i-1]>x){
			int u = a[i] + a[i - 1];
			ans += (u - x);
			a[i] -= (u - x);
		}
	}
	cout << ans;
}

AC

#include <iostream>
#include <cstdio>
#define int long long
using namespace std;

int n, x, a[100010];
int ans = 0;

signed main()
{
	ios::sync_with_stdio(NULL);
	cin >> n >> x;
	for (int i = 1; i <= n; i++){
		cin >> a[i];
	}
	if (a[1]>x){
		ans += (a[1] - x);
		a[1] = x;
	}
	for (int i = 2; i <= n; i++){
		if (a[i]+a[i-1]>x){
			int u = a[i] + a[i - 1];
			ans += (u - x);
			a[i] -= (u - x);
		}
	}
	cout << ans;
}