probability

#include <stdio.h>
#include <stdlib.h>

/* run this program using the console pauser or add your own getch, system("pause") or input loop */

int main(int argc, char *argv[]) {

    int N, M;
    int i, j ,probability;
    char a;
    char b;

    printf("Enter the number of rows: \n");
    scanf(" %d", &N);
    printf("Enter the number of columns: \n");
    scanf(" %d", &M);

    printf("\nEnter first character");
    scanf(" %c", &a);
    printf("\nEnter second characters");
    scanf(" %c", &b);

    while(isdigit(a) || isdigit(b))//check if sine is a digit
    {
        printf("\nEnter two characters");
        scanf(" %c %c", &a, &b);
    }

    for (i= 0; i<N; i++)
    {
        for(j=0; j<M; j++)
        {
            probability=rand()%10; //probability about 1 of 10
            if(probability==1) //probilityy = 1
            {
                printf("%c", a);
            }
            else
                {
                    printf("%c", b);
                }

        }
        printf("\n");
    }

    return 0;
}