[14719] 빗물

let p = readLine()!.split(separator: " ").map{Int(String($0))!}
let (r,c) = (p[0],p[1])
let arr = readLine()!.split(separator: " ").map{Int(String($0))!}
var s = 0
var e = c-1
var maxL = arr[0]
var maxR = arr[e]
var result = 0

while s < e {
    maxL = max(maxL, arr[s])
    maxR = max(maxR, arr[e])
    
    if maxL <= maxR {
        result += maxL - arr[s]
        s += 1
    }else {
        result += maxR - arr[e]
        e -= 1
    }
}
print(result)