Analyze images in group of 10

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Hi, I am currently using this function for image averaging:

cv::Mat avgImg;
avgImg.create(width, height,CV_32FC3);

for(i = 1; i <= N; i++){
  image = imread(fileName.c_str(),0);
  cv::accumulate(image, avgImg);
}

avgImg = avgImg / N;

After this, instead of taking average of all images, I wish to take the average of several groups of 10 images. For example, average of image_1 to image_10, then average of image_11 to image_20 and so on. Is there a function that allows for this?