#include<algorithm>
#include<cmath>
#include<iostream>
#include<string>
#include<cstdio>
using namespace std;
long long candy[10005];
int main() {
	long long n;
	long long x;
	cin >> n >> x;
	for (long long i = 0; i < n; i++) {
		cin >> candy[i];
		scanf("%lld", &candy[i]);
	}
	long long a, b;
	long long ans = 0;
	long long cut;
	for (long long i = 0; i < n - 1; i++) {
		a = candy[i];
		b = candy[i + 1];
		if (a + b > x) {
			cut = a + b - x;
			ans += cut;
			b -= cut;
			if (b < 0) {
				a -= 0 - b;
				b = 0;
			}
			candy[i] = a;
			candy[i + 1] = b;
		}
	}
	cout << ans;
	return 0;
}