Time limit exceeded error when using strlen()?

The following code works as expected, this code prints the character that occurs the most number of times in a string:

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

int main() {

    long int i,a[26]={0},m=0 ,c=0 ;
    char s[1000001] ;
    scanf("%s",s);
    for (i=0;s[i]!='\0';i++){
        a[s[i]-'a']++;
    }
    for ( i=0 ; i<26 ; i++)
        {
            if ( a[i] > m ) {       
            m = a[i] ;
            c = i ;
            }
        }

    printf("%c",'a' + c);
    return 0;
}

but when I use strlen() it causes a time limit error:

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

int main() {

    long int i,a[26]={0},m=0 ,c=0 ;
    char s[1000001] ;
    scanf("%s",s);
    for (i=0;i<strlen(s);i++){
        a[s[i]-'a']++;
    }
    for ( i=0 ; i<26 ; i++)
        {
            if ( a[i] > m ) {       
            m = a[i] ;
            c = i ;
            }
        }

    printf("%c",'a' + c);
    return 0;
}

Where is the problem?