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#include <stdio.h>
#include <string.h>

// Function to check if the string follows the grammar
int checkGrammar(char *str) {
    int len = strlen(str);

    // If the string is too short, it can't match the grammar
    if (len < 2) {
        return 0; // Not valid
    }

    // Check if the first character is 'a' and the last is 'b'
    if (str[0] != 'a' || str[len - 1] != 'b') {
        return 0; // Not valid
    }

    // Check all characters in between
    for (int i = 1; i < len - 1; i++) {
        if (str[i] != 'a' && str[i] != 'b') {
            return 0; // Not valid
        }
    }

    // If all checks pass, the string is valid
    return 1; // Valid
}

int main() {
    char str[100];

    // Get the input string
    printf("Enter a string: ");
    scanf("%s", str);

    // Check the string against the grammar
    if (checkGrammar(str)) {
        printf("The string is valid under the given grammar.\n");
    } else {
        printf("The string is NOT valid under the given grammar.\n");
    }

    return 0;
}