def main():
    import sys
    input = sys.stdin.read().split()
    idx = 0
    N = int(input[idx])
    idx += 1
    C = int(input[idx])
    idx += 1
    
    a = []
    b = []
    for _ in range(N):
        ai = int(input[idx])
        idx += 1
        bi = int(input[idx])
        idx += 1
        a.append(ai)
        b.append(bi)
    
    sum_individual = sum(a[i] + b[i] for i in range(N))
    
    best_single = float('inf')
    for i in range(N):
        pos = i + 1  # 1-based position of worker i
        left = pos - 1
        right = N - pos
        sum_dist = left * (left + 1) // 2 + right * (right + 1) // 2
        cost = b[i] + a[i] * N + C * sum_dist
        if cost < best_single:
            best_single = cost
    
    answer = min(sum_individual, best_single)
    print(answer)

if __name__ == '__main__':
    main()