n, k = map(int, input().split())
p = list(map(int, input().split()))
max = 0
list = [0] * k
if n % 2 == 0:
    roop = int(n / 2)
else:
    roop = int(n / 2) + 1
for i in range(roop):
    wk = 0
    for j in range(i, i + k):
        wk += p[j]
    if max < wk:
        max = wk
        for j in range(i, i + k):
            list[j - i] = p[j]
ans = 0
for i in range(k):
    wk = 0
    for j in range(list[i]):
        wk += j + 1
    ans += float(wk / list[i])
print(ans)

### FIXED CODE ###

n, k = map(int, input().split())
p = list(map(int, input().split()))
max = 0
list = [0] * k
evlist = [1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, 10.0, 10.5, 11.0,
          11.5, 12.0, 12.5, 13.0, 13.5, 14.0, 14.5, 15.0, 15.5, 16.0, 16.5, 17.0, 17.5, 18.0, 18.5, 19.0, 19.5, 20.0,
          20.5, 21.0, 21.5, 22.0, 22.5, 23.0, 23.5, 24.0, 24.5, 25.0, 25.5, 26.0, 26.5, 27.0, 27.5, 28.0, 28.5, 29.0,
          29.5, 30.0, 30.5, 31.0, 31.5, 32.0, 32.5, 33.0, 33.5, 34.0, 34.5, 35.0, 35.5, 36.0, 36.5, 37.0, 37.5, 38.0,
          38.5, 39.0, 39.5, 40.0, 40.5, 41.0, 41.5, 42.0, 42.5, 43.0, 43.5, 44.0, 44.5, 45.0, 45.5, 46.0, 46.5, 47.0,
          47.5, 48.0, 48.5, 49.0, 49.5, 50.0, 50.5, 51.0, 51.5, 52.0, 52.5, 53.0, 53.5, 54.0, 54.5, 55.0, 55.5, 56.0,
          56.5, 57.0, 57.5, 58.0, 58.5, 59.0, 59.5, 60.0, 60.5, 61.0, 61.5, 62.0, 62.5, 63.0, 63.5, 64.0, 64.5, 65.0,
          65.5, 66.0, 66.5, 67.0, 67.5, 68.0, 68.5, 69.0, 69.5, 70.0, 70.5, 71.0, 71.5, 72.0, 72.5, 73.0, 73.5, 74.0,
          74.5, 75.0, 75.5, 76.0, 76.5, 77.0, 77.5, 78.0, 78.5, 79.0, 79.5, 80.0, 80.5, 81.0, 81.5, 82.0, 82.5, 83.0,
          83.5, 84.0, 84.5, 85.0, 85.5, 86.0, 86.5, 87.0, 87.5, 88.0, 88.5, 89.0, 89.5, 90.0, 90.5, 91.0, 91.5, 92.0,
          92.5, 93.0, 93.5, 94.0, 94.5, 95.0, 95.5, 96.0, 96.5, 97.0, 97.5, 98.0, 98.5, 99.0, 99.5, 100.0, 100.5, 101.0,
          101.5, 102.0, 102.5, 103.0, 103.5, 104.0, 104.5, 105.0, 105.5, 106.0, 106.5, 107.0, 107.5, 108.0, 108.5,
          109.0, 109.5, 110.0, 110.5, 111.0, 111.5, 112.0, 112.5, 113.0, 113.5, 114.0, 114.5, 115.0, 115.5, 116.0,
          116.5, 117.0, 117.5, 118.0, 118.5, 119.0, 119.5, 120.0, 120.5, 121.0, 121.5, 122.0, 122.5, 123.0, 123.5,
          124.0, 124.5, 125.0, 125.5, 126.0, 126.5, 127.0, 127.5, 128.0, 128.5, 129.0, 129.5, 130.0, 130.5, 131.0,
          131.5, 132.0, 132.5, 133.0, 133.5, 134.0, 134.5, 135.0, 135.5, 136.0, 136.5, 137.0, 137.5, 138.0, 138.5,
          139.0, 139.5, 140.0, 140.5, 141.0, 141.5, 142.0, 142.5, 143.0, 143.5, 144.0, 144.5, 145.0, 145.5, 146.0,
          146.5, 147.0, 147.5, 148.0, 148.5, 149.0, 149.5, 150.0, 150.5, 151.0, 151.5, 152.0, 152.5, 153.0, 153.5,
          154.0, 154.5, 155.0, 155.5, 156.0, 156.5, 157.0, 157.5, 158.0, 158.5, 159.0, 159.5, 160.0, 160.5, 161.0,
          161.5, 162.0, 162.5, 163.0, 163.5, 164.0, 164.5, 165.0, 165.5, 166.0, 166.5, 167.0, 167.5, 168.0, 168.5,
          169.0, 169.5, 170.0, 170.5, 171.0, 171.5, 172.0, 172.5, 173.0, 173.5, 174.0, 174.5, 175.0, 175.5, 176.0,
          176.5, 177.0, 177.5, 178.0, 178.5, 179.0, 179.5, 180.0, 180.5, 181.0, 181.5, 182.0, 182.5, 183.0, 183.5,
          184.0, 184.5, 185.0, 185.5, 186.0, 186.5, 187.0, 187.5, 188.0, 188.5, 189.0, 189.5, 190.0, 190.5, 191.0,
          191.5, 192.0, 192.5, 193.0, 193.5, 194.0, 194.5, 195.0, 195.5, 196.0, 196.5, 197.0, 197.5, 198.0, 198.5,
          199.0, 199.5, 200.0, 200.5, 201.0, 201.5, 202.0, 202.5, 203.0, 203.5, 204.0, 204.5, 205.0, 205.5, 206.0,
          206.5, 207.0, 207.5, 208.0, 208.5, 209.0, 209.5, 210.0, 210.5, 211.0, 211.5, 212.0, 212.5, 213.0, 213.5,
          214.0, 214.5, 215.0, 215.5, 216.0, 216.5, 217.0, 217.5, 218.0, 218.5, 219.0, 219.5, 220.0, 220.5, 221.0,
          221.5, 222.0, 222.5, 223.0, 223.5, 224.0, 224.5, 225.0, 225.5, 226.0, 226.5, 227.0, 227.5, 228.0, 228.5,
          229.0, 229.5, 230.0, 230.5, 231.0, 231.5, 232.0, 232.5, 233.0, 233.5, 234.0, 234.5, 235.0, 235.5, 236.0,
          236.5, 237.0, 237.5, 238.0, 238.5, 239.0, 239.5, 240.0, 240.5, 241.0, 241.5, 242.0, 242.5, 243.0, 243.5,
          244.0, 244.5, 245.0, 245.5, 246.0, 246.5, 247.0, 247.5, 248.0, 248.5, 249.0, 249.5, 250.0, 250.5, 251.0,
          251.5, 252.0, 252.5, 253.0, 253.5, 254.0, 254.5, 255.0, 255.5, 256.0, 256.5, 257.0, 257.5, 258.0, 258.5,
          259.0, 259.5, 260.0, 260.5, 261.0, 261.5, 262.0, 262.5, 263.0, 263.5, 264.0, 264.5, 265.0, 265.5, 266.0,
          266.5, 267.0, 267.5, 268.0, 268.5, 269.0, 269.5, 270.0, 270.5, 271.0, 271.5, 272.0, 272.5, 273.0, 273.5,
          274.0, 274.5, 275.0, 275.5, 276.0, 276.5, 277.0, 277.5, 278.0, 278.5, 279.0, 279.5, 280.0, 280.5, 281.0,
          281.5, 282.0, 282.5, 283.0, 283.5, 284.0, 284.5, 285.0, 285.5, 286.0, 286.5, 287.0, 287.5, 288.0, 288.5,
          289.0, 289.5, 290.0, 290.5, 291.0, 291.5, 292.0, 292.5, 293.0, 293.5, 294.0, 294.5, 295.0, 295.5, 296.0,
          296.5, 297.0, 297.5, 298.0, 298.5, 299.0, 299.5, 300.0, 300.5, 301.0, 301.5, 302.0, 302.5, 303.0, 303.5,
          304.0, 304.5, 305.0, 305.5, 306.0, 306.5, 307.0, 307.5, 308.0, 308.5, 309.0, 309.5, 310.0, 310.5, 311.0,
          311.5, 312.0, 312.5, 313.0, 313.5, 314.0, 314.5, 315.0, 315.5, 316.0, 316.5, 317.0, 317.5, 318.0, 318.5,
          319.0, 319.5, 320.0, 320.5, 321.0, 321.5, 322.0, 322.5, 323.0, 323.5, 324.0, 324.5, 325.0, 325.5, 326.0,
          326.5, 327.0, 327.5, 328.0, 328.5, 329.0, 329.5, 330.0, 330.5, 331.0, 331.5, 332.0, 332.5, 333.0, 333.5,
          334.0, 334.5, 335.0, 335.5, 336.0, 336.5, 337.0, 337.5, 338.0, 338.5, 339.0, 339.5, 340.0, 340.5, 341.0,
          341.5, 342.0, 342.5, 343.0, 343.5, 344.0, 344.5, 345.0, 345.5, 346.0, 346.5, 347.0, 347.5, 348.0, 348.5,
          349.0, 349.5, 350.0, 350.5, 351.0, 351.5, 352.0, 352.5, 353.0, 353.5, 354.0, 354.5, 355.0, 355.5, 356.0,
          356.5, 357.0, 357.5, 358.0, 358.5, 359.0, 359.5, 360.0, 360.5, 361.0, 361.5, 362.0, 362.5, 363.0, 363.5,
          364.0, 364.5, 365.0, 365.5, 366.0, 366.5, 367.0, 367.5, 368.0, 368.5, 369.0, 369.5, 370.0, 370.5, 371.0,
          371.5, 372.0, 372.5, 373.0, 373.5, 374.0, 374.5, 375.0, 375.5, 376.0, 376.5, 377.0, 377.5, 378.0, 378.5,
          379.0, 379.5, 380.0, 380.5, 381.0, 381.5, 382.0, 382.5, 383.0, 383.5, 384.0, 384.5, 385.0, 385.5, 386.0,
          386.5, 387.0, 387.5, 388.0, 388.5, 389.0, 389.5, 390.0, 390.5, 391.0, 391.5, 392.0, 392.5, 393.0, 393.5,
          394.0, 394.5, 395.0, 395.5, 396.0, 396.5, 397.0, 397.5, 398.0, 398.5, 399.0, 399.5, 400.0, 400.5, 401.0,
          401.5, 402.0, 402.5, 403.0, 403.5, 404.0, 404.5, 405.0, 405.5, 406.0, 406.5, 407.0, 407.5, 408.0, 408.5,
          409.0, 409.5, 410.0, 410.5, 411.0, 411.5, 412.0, 412.5, 413.0, 413.5, 414.0, 414.5, 415.0, 415.5, 416.0,
          416.5, 417.0, 417.5, 418.0, 418.5, 419.0, 419.5, 420.0, 420.5, 421.0, 421.5, 422.0, 422.5, 423.0, 423.5,
          424.0, 424.5, 425.0, 425.5, 426.0, 426.5, 427.0, 427.5, 428.0, 428.5, 429.0, 429.5, 430.0, 430.5, 431.0,
          431.5, 432.0, 432.5, 433.0, 433.5, 434.0, 434.5, 435.0, 435.5, 436.0, 436.5, 437.0, 437.5, 438.0, 438.5,
          439.0, 439.5, 440.0, 440.5, 441.0, 441.5, 442.0, 442.5, 443.0, 443.5, 444.0, 444.5, 445.0, 445.5, 446.0,
          446.5, 447.0, 447.5, 448.0, 448.5, 449.0, 449.5, 450.0, 450.5, 451.0, 451.5, 452.0, 452.5, 453.0, 453.5,
          454.0, 454.5, 455.0, 455.5, 456.0, 456.5, 457.0, 457.5, 458.0, 458.5, 459.0, 459.5, 460.0, 460.5, 461.0,
          461.5, 462.0, 462.5, 463.0, 463.5, 464.0, 464.5, 465.0, 465.5, 466.0, 466.5, 467.0, 467.5, 468.0, 468.5,
          469.0, 469.5, 470.0, 470.5, 471.0, 471.5, 472.0, 472.5, 473.0, 473.5, 474.0, 474.5, 475.0, 475.5, 476.0,
          476.5, 477.0, 477.5, 478.0, 478.5, 479.0, 479.5, 480.0, 480.5, 481.0, 481.5, 482.0, 482.5, 483.0, 483.5,
          484.0, 484.5, 485.0, 485.5, 486.0, 486.5, 487.0, 487.5, 488.0, 488.5, 489.0, 489.5, 490.0, 490.5, 491.0,
          491.5, 492.0, 492.5, 493.0, 493.5, 494.0, 494.5, 495.0, 495.5, 496.0, 496.5, 497.0, 497.5, 498.0, 498.5,
          499.0, 499.5, 500.0, 500.5]
wk = 0
start = 0
for i in range(n - (k - 1)):
    if wk == 0:
        for j in range(i, i + k):
            wk += p[j]
    else:
        wk += p[i + k - 1]
        wk -= p[i - 1]
    if max < wk:
        max = wk
        start = i
for i in range(start, start + k):
    list[i - start] = p[i]
ans = 0
for i in range(k):
    ans += evlist[list[i] - 1]
print(ans)