import sys

def main():
    import sys
    N, C = map(int, sys.stdin.readline().split())
    ab = [tuple(map(int, sys.stdin.readline().split())) for _ in range(N)]
    a = [x[0] for x in ab]
    b = [x[1] for x in ab]
    
    # Calculate the cost if all statues are built by craftsman i
    min_all = float('inf')
    for i in range(N):
        total = b[i] + a[i] * N
        sum_dist = 0
        for j in range(N):
            sum_dist += abs(i - j)
        total += sum_dist * C
        if total < min_all:
            min_all = total
    
    # Initialize DP with the minimum of single craftsman
    dp = [float('inf')] * (N + 1)
    dp[0] = 0
    for r in range(1, N+1):
        for l in range(r-1, max(r-4, -1), -1):
            current_min = float('inf')
            for k in range(l, r):
                cost = b[k] + a[k] * (r - l)
                sum_d = 0
                for x in range(l+1, r+1):
                    sum_d += abs(k + 1 - x)
                cost += sum_d * C
                if dp[l] + cost < current_min:
                    current_min = dp[l] + cost
            if current_min < dp[r]:
                dp[r] = current_min
    
    result = min(min_all, dp[N])
    print(result)

if __name__ == "__main__":
    main()