Expected identifier or '(' in simple for-loop in C

I write functiton, that check is string contain only letters or not. If i declare n outside the loop:

int n = strlen(str);
for (int i = 0; i < n; i++)

it has no errors and works great, but if i move n declaration inside:

for (int i = 0, int n = strlen(str); i < n; i++)

i have errors:

vigenere.c:71:21: error: expected identifier or '('
    for (int i = 0, int n = strlen(str); i < n; i++)
                    ^
vigenere.c:71:21: error: expected ';' in 'for' statement specifier
vigenere.c:71:21: error: expected expression
vigenere.c:71:46: error: use of undeclared identifier 'n'
    for (int i = 0, int n = strlen(str); i < n; i++)
                                             ^
vigenere.c:71:47: error: expected ')'
    for (int i = 0, int n = strlen(str); i < n; i++)
                                              ^
vigenere.c:71:9: note: to match this '('
    for (int i = 0, int n = strlen(str); i < n; i++)
        ^
vigenere.c:71:49: error: use of undeclared identifier 'i'
    for (int i = 0, int n = strlen(str); i < n; i++)
                                                ^
6 errors generated.

Why? I have the same loop in main() and it's work great. Can somebody explain me the problem? Thanks!

Fuction entirely:

int is_alpha_string(string str)
{
    //for (int i = 0, int n = strlen(str); i < n; i++)
    int n = strlen(str);
    for (int i = 0; i < n; i++)
    {
        if (isalpha(str[i]) == 0)
        {
            return 0;
        }
    }

    return 1;
}

Change:

for (int i = 0, int n = strlen(str); i < n; i++)

to:

for (int i = 0, n = strlen(str); i < n; i++)

(Note that the syntax here is much the same as it would be for any declaration of multiple variables with the same type, regardless of whether it's in a for loop or not.)