def main():
    import sys
    input = sys.stdin.read().split()
    idx = 0
    N, C = int(input[idx]), int(input[idx+1])
    idx +=2
    ab = []
    sum_individual = 0
    for _ in range(N):
        a = int(input[idx])
        b = int(input[idx+1])
        ab.append((a, b))
        sum_individual += a + b
        idx +=2
    
    min_full = float('inf')
    for i in range(N):
        a, b = ab[i]
        pos = i + 1  # 1-based
        left = pos - 1
        left_sum = left * (left + 1) // 2
        right = N - pos
        right_sum = right * (right + 1) // 2
        total_travel = C * (left_sum + right_sum)
        total_cost = b + a * N + total_travel
        if total_cost < min_full:
            min_full = total_cost
    
    print(min(min_full, sum_individual))

if __name__ == '__main__':
    main()